Fix issues in math implementation

[elichai]

None of these issues should affect consensus

1. Incorrect overflow bit in Uint implementation (We don't use the overflowing_* variants, and the Mul trait impl ignores the overflow bit in release mode (it uses a debug_assert) so nothing should really be affected).
2. Incorrect .size_hint() iterator impl for the iterator returned at .iter_be_bits() when combined with the overriden .nth() function.
3. Incorrect as_f64 when used on Uint3072 on values above ~2^1024 (before it could return garbage values, now it will always return Inf in those cases)

First commit just makes the construct_macro work in the fuzzing infra again.

Second commit improves the tests and fuzzing to catch these errors:

Test Failures

failures:

---- tests::as_f64_u3072_saturates_to_positive_infinity stdout ----

thread 'tests::as_f64_u3072_saturates_to_positive_infinity' (15627144) panicked at math/src/lib.rs:205:9:
assertion `left == right` failed
  left: -inf
 right: inf
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- uint::tests::test_mul_overflow stdout ----

thread 'uint::tests::test_mul_overflow' (15627149) panicked at math/src/uint.rs:1207:9:
assertion `left == right` failed
  left: true
 right: false

---- uint::tests::test_iter_be_bits stdout ----

thread 'uint::tests::test_iter_be_bits' (15627147) panicked at math/src/uint.rs:1045:5:
attempt to subtract with overflow


failures:
    tests::as_f64_u3072_saturates_to_positive_infinity
    uint::tests::test_iter_be_bits
    uint::tests::test_mul_overflow

test result: FAILED. 7 passed; 3 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.52s

Fuzzing Failures

thread '<unnamed>' (15631338) panicked at fuzz_targets/u128.rs:73:5:
assertion `left == right` failed: native: 654229641859875823987855318113583104, native2: 254422585063317791287472979454197760
  left: false
 right: true
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
==87225== ERROR: libFuzzer: deadly signal
    #0 0x0001016c32f4 in __sanitizer_print_stack_trace+0x28 (librustc-nightly_rt.asan.dylib:arm64+0x872f4)
    #1 0x0001009e5860 in fuzzer::PrintStackTrace()+0x30 (u128:arm64+0x100571860)
    #2 0x0001009d9a6c in fuzzer::Fuzzer::CrashCallback()+0x54 (u128:arm64+0x100565a6c)
    #3 0x0001882f1760 in _sigtramp+0x34 (libsystem_platform.dylib:arm64e+0x3760)
    #4 0x0001882e7884 in pthread_kill+0x124 (libsystem_pthread.dylib:arm64e+0x6884)
    #5 0x0001881ec84c in abort+0x78 (libsystem_c.dylib:arm64e+0x7984c)
    #6 0x000100a3fca4 in _RNvNtNtNtCsdstvSoFip2a_3std3sys3pal4unix14abort_internal mod.rs:305
    #7 0x000100a3c724 in _RNvNtCsdstvSoFip2a_3std7process5abort process.rs:2533
    #8 0x000100a12b34 in _RNCNvCs1acPNacv4I9_13libfuzzer_sys10initialize0B3_ lib.rs:94
    #9 0x00010092f8d4 in _RNvXsv_NtCs7ZSLYPp39HD_5alloc5boxedINtB5_3BoxDG0_INtNtNtCsdMl2AJbG7IL_4core3ops8function2FnTRL1_INtNtCsdstvSoFip2a_3std5panic13PanicHookInfoL0_EEEp6OutputuNtNtBS_6marker4SyncNtB2v_4SendEL_EIBM_TRB1x_EE4callB1B_ boxed.rs:2264
    #10 0x00010092f8d4 in _RNvNtCsdstvSoFip2a_3std9panicking15panic_with_hook panicking.rs:833
    #11 0x00010094e960 in _RNCNvNtCsdstvSoFip2a_3std9panicking13panic_handler0B5_ panicking.rs:698
    #12 0x00010094e91c in _RINvNtNtCsdstvSoFip2a_3std3sys9backtrace26___rust_end_short_backtraceNCNvNtB6_9panicking13panic_handler0zEB6_ backtrace.rs:182
    #13 0x00010094efa4 in _RNvCscPXrf1V1aal_7___rustc17rust_begin_unwind panicking.rs:689
    #14 0x000100a0d17c in _RNvNtCsdMl2AJbG7IL_4core9panicking9panic_fmt panicking.rs:80
    #15 0x000100a0d154 in _RNvNtCsdMl2AJbG7IL_4core9panicking19assert_failed_inner panicking.rs
    #16 0x000100a4b9b8 in _RINvNtCsdMl2AJbG7IL_4core9panicking13assert_failedbbECs5gxNaq7wCLv_5tokio panicking.rs:394
    #17 0x000100479e14 in assert_op_with_overflow<fn(u128::Uint128, u128::Uint128) -> (u128::Uint128, bool), fn(u128, u128) -> (u128, bool)> u128.rs:58
    #18 0x000100479e14 in _RNvNvCs6kb0rnmNY2n_4u1281__19___libfuzzer_sys_run u128.rs:73
    #19 0x000100480620 in rust_fuzzer_test_input lib.rs:276
    #20 0x000100519918 in {closure#0} lib.rs:62
    #21 0x000100519918 in _RINvNvNtCsdstvSoFip2a_3std9panicking12catch_unwind7do_callNCNvCs1acPNacv4I9_13libfuzzer_sys15test_input_wrap0lEBY_ panicking.rs:581
    #22 0x000100519844 in __rust_try.7824+0x18 (u128:arm64+0x1000a5844)
    #23 0x00010051adc4 in catch_unwind<i32, libfuzzer_sys::test_input_wrap::{closure_env#0}> panicking.rs:544
    #24 0x00010051adc4 in catch_unwind<libfuzzer_sys::test_input_wrap::{closure_env#0}, i32> panic.rs:359
    #25 0x00010051adc4 in LLVMFuzzerTestOneInput lib.rs:60
    #26 0x0001009db314 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long)+0x150 (u128:arm64+0x100567314)
    #27 0x0001009da9c0 in fuzzer::Fuzzer::RunOne(unsigned char const*, unsigned long, bool, fuzzer::InputInfo*, bool, bool*)+0x48 (u128:arm64+0x1005669c0)
    #28 0x0001009dcb5c in fuzzer::Fuzzer::ReadAndExecuteSeedCorpora(std::__1::vector<fuzzer::SizedFile, std::__1::allocator<fuzzer::SizedFile>>&)+0x818 (u128:arm64+0x100568b5c)
    #29 0x0001009dccf4 in fuzzer::Fuzzer::Loop(std::__1::vector<fuzzer::SizedFile, std::__1::allocator<fuzzer::SizedFile>>&)+0x88 (u128:arm64+0x100568cf4)
    #30 0x0001009fb8d4 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long))+0x1abc (u128:arm64+0x1005878d4)
    #31 0x000100a08338 in main+0x24 (u128:arm6

The rest of the commits fix those issues