[WIP] File utilities using WABT

[tshort]

This add a few methods for file IO, particularly for writing wast and wasm files.

This uses WABT with binaries from WABTBuilder.

I've only really tested this on Linux.

It's marked as WIP because there are no tests, yet.

[MikeInnes]

LGTM. I can easily test on OS X if you need me to.

[tshort]

No hurry on testing on OS X, @MikeInnes.