Refactor code to make it more amenable to cross compilation

[gbaraldi]

Also cleanup some of it.

This doesn't allow for actual cross-compilation because we need the compilers, and implementing that is basically implementing BinaryBuilder, but maybe we should add an option to pass cc.

[tshort]

Plus one from me. Could you add an example in the docs or maybe even a test of a cross compilation? Cross compiling to WebAssembly should work because that's built into the compiler.

Do any of the compilers in Yggdrasil provide cross compilation?

[gbaraldi]

Yggy has tons of cross compilers, but they only run in linux.

[brenhinkeller]

Cool! We should make sure we're being consistent about cflags/c_flags and etc. but otherwise LGTM

[MasonProtter]

Looks good to me. Would you mind bumping the major version and then I think this is ready to go. Fixes for nightly can be done later once the interfaces and such in nightly stabilize a bit.

[tshort]

Sorry. No one pulled the trigger here. Anything else we need to do here?