Add very limited support for `#[builder(const)]` #929

Job	Run time
version-sync	5s
cargo-lock	7s
compilation-benchmarks	2m 16s
runtime-benchmarks (args_3)	1m 22s
test-msrv (ubuntu)	4m 26s
runtime-benchmarks (args_5)	1m 33s
test-msrv (windows)	5m 36s
runtime-benchmarks (args_10_alloc)	1m 21s
test-msrv (macos)	3m 16s
runtime-benchmarks (args_10_structs)	1m 28s
runtime-benchmarks (args_10)	1m 27s
runtime-benchmarks (args_20)	1m 26s
cargo-doc (ubuntu)	32s
cargo-fmt	22s
cargo-doc (windows)	3m 10s
cargo-doc (macos)	43s
cargo-machete	3s
cargo-miri (--locked)	1m 56s
cargo-miri	1m 50s
prettier	8s
taplo-fmt	4s
test-stable (ubuntu, --locked)	1m 19s
test-unstable (beta, --locked)	33s
test-stable (ubuntu)	1m 17s
typos	5s
test-unstable (beta)	31s
test-stable (windows, --locked)	3m 40s
website-build	20s
test-unstable (nightly, --locked)	28s
test-stable (windows)	3m 51s
test-unstable (nightly)	27s
test-stable (macos, --locked)	1m 19s
test-stable (macos)	1m 16s
	48m 17s

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