Move bswap to builtins. Remove bswap16

[MaxGraey]

• Remove bswap from polyfills.ts and expose it as compiler's builtin.
• Remove bswap16. We didn't use it and I can't imagine where it would be needed. So removing it and don't mark this PR as breaking changed due to its mostly internal change.
• Add Type#toUnsigned helper which try to force type to unsigned. Useful for sign agnostic operations, especially generic types, due to reduce polymorphic noise.
• Remove polyfills.ts and according tests. Move bswap tests to tests/builtins

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[MaxGraey]

It might even be worth transferring this to builtins.ts and generating it immediately to binaryen IR.