Get and Set Globals

[RoyalIcing]

Add Wasmex.Instance.get_global_value/3 and Wasmex.Instance.set_global_value/4 to read and write globals.

I don’t believe languages like Rust or C allow exporting globals, but in raw WebAssembly this is very much possible:

(module
  (global (export "meaning_of_life") i32 (i32.const 42))
  (global (export "count") (mut i64) (i64.const 0))
)

I’ve followed what appears to be the convention of returning string errors. If you like I can make them atoms like {:error, :not_found} or {:error, :immutable}.

[tessi]

Wow, this is great 💜 love the change, fits perfectly to existing conventions, implements a missing festure and reads really well.

Thank you.

If you like, you may add your work to the changelog :)