wasi: define wasi implementations + zware-gen uses these definitions

[malcolmstill]

Description

• Modifies zware-gen so no function stubs are created for wasi functions (module wasi_snapshot_preview1)
• Implements enough wasi to run doom!
  • Luckily, as part of ditching the C++ implementation of the zig compiler, Andrew / Jacob have done the hard work and showed how to implement a bunch of these functions in https://github.com/andrewrk/zig-wasi
  • Adds an api (via Instance) for adding preopens and forwarding on args
  • Whilst the interface is on Instance, access within the wasi functions is via VirtualMachine directly (because remember vm.inst (potentially) changes to different Instances during execution...the non-invoking Instances will not have preopens / args

Screen.Recording.2023-07-17.at.02.18.40.mov

Follow ups

• Need to implement more wasi (I currently comment out some code that reads env vars in the Doom source code which would otherwise pull in the wasi env functions)
  • How do we test this? Can I get a simple .c program that requires the complete wasi?