Generate strerror from witx; tweak Display for WasiError (#832)

This commit introduces two small changes:
* it adds `gen_errno_strerror` to `wig` crate which generates a
  `strerror` function for `__wasi_errno_t` directly from `*.witx`,
  similarly to how it's done in the `wasi` crate
* it tweaks `WasiError` type to include the error message generated
  with `strerror` when displaying the error

crates/wasi-common/src/error.rs

@@ -8,7 +8,7 @@ use thiserror::Error;
 
 #[derive(Clone, Copy, Debug, Error, Eq, PartialEq)]
 #[repr(u16)]
-#[error("{:?}", self)]
+#[error("{:?} ({})", self, wasi::strerror(*self as wasi::__wasi_errno_t))]
 pub enum WasiError {
     ESUCCESS = wasi::__WASI_ERRNO_SUCCESS,
     E2BIG = wasi::__WASI_ERRNO_2BIG,