Shrink Crystal::System.print_error's output size

[HertzDevil]

Crystal::System is by far the single largest LLVM module when compiling a blank source file, even though all the module does by itself is defining .print_error and friends. On my machine, with debug information stripped, C-rystal5858S-ystem.o0.bc is 213.4 KiB big, compared to _main.o0.bc's 101.7 KiB. Disassembling the bytecode back to LLVM IR produces a monstrosity with 33k lines. This PR brings the numbers down to 48.0 KiB and 6.1k lines, while slightly improving performance, using the following tricks:

• The type of .as?(T) is always T? and does not perform intersection, so even simple types like Int32 are upcast into the whole Int::Primitive?, leading to a lot of redundant downcasts later. A simple is_a? will suffice as a type filter in read_arg. (I believe this is mentioned somewhere but couldn't find it)
• In .to_int_slice, the num variable is cast into an Int32 | UInt32 | Int64 | UInt64, and each subsequent line dispatches over that union. The fix here is to split the rest of the body into a separate method, and call it with each variant of the union. This form of dispatching is akin to rewriting .to_int_slice as an instance method on the integers.
• .to_int_slice is now non-yielding, as the inlining added too much bloat. The caller is responsible for preparing a suitably sized buffer.

Additionally, this reduces the time for the bytecode generation phase from an average of 0.35s down to 0.26s.

[ysbaddaden]

Terrific 👍

I've been reusing the to_int_slice calls in Crystal.trace (https://github.com/crystal-lang/crystal/blob/master/src/crystal/tracing.cr) so we'll have to fix the calls there too.