direct-wasm32-wasip1-wasm backend

[alex-s168]

not that hard to do since we don't really need to care about optimizations (because all good wasm runtimes have good JITs), and wasm is relatively simple.

only things that might make this a bit annoying:

• wasm binary file writer

blockers:

• split lib #1313
• target "triples" #1452
• Pave the way for backend plugins #1190
• ( Replace C backend plugin with just library definitions #1299 )
• ( way to specify C export name of non-generic functions #1246 )