[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/fs/file.rs

@@ -1,5 +1,6 @@
 use crate::fs::Metadata;
-use crate::{host, hostcalls, hostcalls_impl, wasi, Result, WasiCtx};
+use crate::wasi::{self, WasiResult};
+use crate::{host, hostcalls, hostcalls_impl, WasiCtx};
 use std::io;
 
 /// A reference to an open file on the filesystem.
@@ -34,7 +35,7 @@ impl<'ctx> File<'ctx> {
     /// This corresponds to [`std::fs::File::sync_all`].
     ///
     /// [`std::fs::File::sync_all`]: https://doc.rust-lang.org/std/fs/struct.File.html#method.sync_all
-    pub fn sync_all(&self) -> Result<()> {
+    pub fn sync_all(&self) -> WasiResult<()> {
         unsafe {
             hostcalls_impl::fd_sync(self.ctx, &mut [], self.fd)?;
         }
@@ -47,7 +48,7 @@ impl<'ctx> File<'ctx> {
     /// This corresponds to [`std::fs::File::sync_data`].
     ///
     /// [`std::fs::File::sync_data`]: https://doc.rust-lang.org/std/fs/struct.File.html#method.sync_data
-    pub fn sync_data(&self) -> Result<()> {
+    pub fn sync_data(&self) -> WasiResult<()> {
         unsafe {
             hostcalls_impl::fd_datasync(self.ctx, &mut [], self.fd)?;
         }
@@ -60,7 +61,7 @@ impl<'ctx> File<'ctx> {
     /// This corresponds to [`std::fs::File::set_len`].
     ///
     /// [`std::fs::File::set_len`]: https://doc.rust-lang.org/std/fs/struct.File.html#method.set_len
-    pub fn set_len(&self, size: u64) -> Result<()> {
+    pub fn set_len(&self, size: u64) -> WasiResult<()> {
         unsafe {
             hostcalls_impl::fd_filestat_set_size(self.ctx, &mut [], self.fd, size)?;
         }
@@ -72,7 +73,7 @@ impl<'ctx> File<'ctx> {
     /// This corresponds to [`std::fs::File::metadata`].
     ///
     /// [`std::fs::File::metadata`]: https://doc.rust-lang.org/std/fs/struct.File.html#method.metadata
-    pub fn metadata(&self) -> Result<Metadata> {
+    pub fn metadata(&self) -> WasiResult<Metadata> {
         Ok(Metadata {})
     }
 }