Refactor and combine all FileType structs in yanix

This commit does a bit of everything: refactors bits here and there,
fixes a bug discovered in another #701, and combines all structs that
we used in `yanix` and `wasi-common` crates to represent file types
on *nix into one struct, `yanix::file::FileType`.

Up until now, in `yanix`, we've had two separate structs used to
represent file types on the host: `yanix::dir::FileType` and
`yanix::file::SFlags` (well, not quite, but that was its main use).
They both were used in different context (the former when parsing
`dirent` struct, and the latter when parsing `stat` struct), they
were C-compatible (as far as their representation goes), and as it
turns out, they shared possible enumeration values. This commit
combines them both into an idiomatic Rust enum with the caveat that
it is now *not* C-compatible, however, I couldn't find a single use
where that would actually matter, and even if it does in the future,
we can simply add appropriate impl methods.

The combine `yanix::file::FileType` struct can be constructed in two
ways: 1) either from `stat.st_mode` value (and while we're here,
now it's done correctly according to POSIX which fixes the bug mentioned
in VFS impl PR #701), or 2) from `dirent.d_type` value. Also, since we now
have one struct for representing both contexts, this cleans up nicely
a lot of duplicated code in `host` module.

crates/wasi-common/yanix/src/dir.rs

@@ -1,4 +1,5 @@
 use crate::{
+    file::FileType,
     sys::dir::{iter_impl, EntryImpl},
     Errno, Result,
 };
@@ -84,7 +85,7 @@ impl Entry {
 
     /// Returns the type of this directory entry.
     pub fn file_type(&self) -> FileType {
-        FileType::from_raw(self.0.d_type)
+        FileType::from_dirent_d_type(self.0.d_type)
     }
 }
 
@@ -99,47 +100,6 @@ impl SeekLoc {
     }
 }
 
-#[derive(Clone, Copy, Debug)]
-#[repr(u8)]
-pub enum FileType {
-    CharacterDevice = libc::DT_CHR,
-    Directory = libc::DT_DIR,
-    BlockDevice = libc::DT_BLK,
-    RegularFile = libc::DT_REG,
-    Symlink = libc::DT_LNK,
-    Fifo = libc::DT_FIFO,
-    Socket = libc::DT_SOCK,
-    Unknown = libc::DT_UNKNOWN,
-}
-
-impl FileType {
-    pub fn from_raw(file_type: u8) -> Self {
-        match file_type {
-            libc::DT_CHR => Self::CharacterDevice,
-            libc::DT_DIR => Self::Directory,
-            libc::DT_BLK => Self::BlockDevice,
-            libc::DT_REG => Self::RegularFile,
-            libc::DT_LNK => Self::Symlink,
-            libc::DT_SOCK => Self::Socket,
-            libc::DT_FIFO => Self::Fifo,
-            /* libc::DT_UNKNOWN */ _ => Self::Unknown,
-        }
-    }
-
-    pub fn to_raw(&self) -> u8 {
-        match self {
-            Self::CharacterDevice => libc::DT_CHR,
-            Self::Directory => libc::DT_DIR,
-            Self::BlockDevice => libc::DT_BLK,
-            Self::RegularFile => libc::DT_REG,
-            Self::Symlink => libc::DT_LNK,
-            Self::Socket => libc::DT_SOCK,
-            Self::Fifo => libc::DT_FIFO,
-            Self::Unknown => libc::DT_UNKNOWN,
-        }
-    }
-}
-
 #[derive(Debug)]
 pub struct DirIter<T: Deref<Target = Dir>>(T);