Implement WASI

[malcolmstill]

Figure out how WASI works and implement it.

[malcolmstill]

Enough WASI to run doom was implemented in #206.

We want a complete implementation. To that end we need to:
a) implement the whole spec
b) test our implementation...there seems to be a testsuite that we can use here: https://github.com/WebAssembly/wasi-testsuite