[wasi-common]: clean up error handling (#1253)

* Introduce WasiCtxBuilderError error type

`WasiCtxBuilderError` is the `wasi-common` client-facing error type
which is exclusively thrown when building a new `WasiCtx` instance.
As such, building such an instance should not require the client to
understand different WASI errno values as was assumed until now.

This commit is a first step at streamlining error handling in
`wasi-common` and makes way for the `wiggle` crate.

When adding the `WasiCtxBuilderError`, I've had to do two things of
notable importance:
1. I've removed a couple of `ok_or` calls in `WasiCtxBuilder::build`
   and replaced them with `unwrap`s, following the same pattern in
   different builder methods above. This is fine since we _always_
   operate on non-empty `Option`s in `WasiCtxBuilder` thus `unwrap`ing
   will never fail. On the other hand, this might be a good opportunity
   to rethink the structure of our builder, and how we good remove
   the said `Option`s especially since we always populate them with
   empty containers to begin with. I understand this is to make
   chaining of builder methods easier which take and return `&mut self`
   and the same applies to `WasiCtxBuilder::build(&mut self)` method,
   but perhaps it would more cleanly signal the intentions if we simply
   moved `WasiCtxBuilder` instance around. Food for thought!
2. Methods specific to determining rights of passed around `std::fs::File`
   objects when populating `WasiCtx` `FdEntry` entities now return
   `io::Error` directly so that we can reuse them in `WasiCtxBuilder` methods
   (returning `WasiCtxBuilderError` error type), and in syscalls
   (returning WASI errno).

* Return WasiError directly in syscalls

Also, removes `error::Error` type altogether. Now, `io::Error` and
related are automatically converted to their corresponding WASI
errno value encapsulated as `WasiError`.

While here, it made sense to me to move `WasiError` to `wasi` module
which will align itself well with the upcoming changes introduced
by `wiggle`. To different standard `Result` from WASI specific, I've
created a helper alias `WasiResult` also residing in `wasi` module.

* Update wig

* Add from ffi::NulError and pass context to NotADirectory

* Add dummy commit to test CI

crates/wasi-common/src/host.rs

@@ -5,7 +5,6 @@
 #![allow(non_snake_case)]
 
 use crate::wasi::*;
-use crate::{Error, Result};
 use std::{convert::TryInto, io, mem, slice};
 use wig::witx_host_types;
 
@@ -67,11 +66,13 @@ pub struct Dirent {
 impl Dirent {
     /// Serialize the directory entry to the format define by `__wasi_fd_readdir`,
     /// so that the serialized entries can be concatenated by the implementation.
-    pub fn to_wasi_raw(&self) -> Result<Vec<u8>> {
+    pub fn to_wasi_raw(&self) -> WasiResult<Vec<u8>> {
         let name = self.name.as_bytes();
         let namlen = name.len();
         let dirent_size = mem::size_of::<__wasi_dirent_t>();
-        let offset = dirent_size.checked_add(namlen).ok_or(Error::EOVERFLOW)?;
+        let offset = dirent_size
+            .checked_add(namlen)
+            .ok_or(WasiError::EOVERFLOW)?;
 
         let mut raw = Vec::<u8>::with_capacity(offset);
         raw.resize(offset, 0);