WASI basic support

[arnoox]

• WASI trait definitions in runtime (WasiFd, WasiPath, WasiClock, WasiRandom)
• Transpiler recognizes WASI import names → maps to WASI traits
• Reference host implementation (at least fd_write, fd_read, proc_exit)
• End-to-end test: C printf hello world → Wasm → Rust → run