DMD Frontend working in WebAssembly

[hatf0]

Hi all,

I've just managed to get the full DMD front-end to work in 
WebAssembly (with skoppe's druntime fork). This doesn't do 
code-gen or anything (but it potentially could?), and has some 
OS-specific functionality stubbed out. No clue about GC -- 
haven't run into that issue, haven't thought about it yet!

You can find my work here if you're interested: 
https://github.com/hatf0/dmd-fe-wasm-wrapper

This repo also serves as a semi-decent guide on how to get 
started building files that target WebAssembly (specifically 
WASI), if you're into that.