feat: add support for wasi (#470)

This allows path conversions there to be just as efficient as on unix.

This was adopted from [a PR in the
hexlix-editor](https://github.com/helix-editor/helix/pull/3890/files#diff-504515b66023120e75a921cd56a932aed76c0ff62593fbb69d92e0ef65089501R47).

git-path/src/convert.rs

@@ -41,6 +41,11 @@ pub fn try_into_bstr<'a>(path: impl Into<Cow<'a, Path>>) -> Result<Cow<'a, BStr>
                 use std::os::unix::ffi::OsStringExt;
                 path.into_os_string().into_vec().into()
             };
+            #[cfg(target_os = "wasi")]
+            let p: BString = {
+                use std::os::wasi::ffi::OsStringExt;
+                path.into_os_string().into_vec().into()
+            };
             #[cfg(not(unix))]
             let p: BString = path.into_os_string().into_string().map_err(|_| Utf8Error)?.into();
             p
@@ -51,6 +56,11 @@ pub fn try_into_bstr<'a>(path: impl Into<Cow<'a, Path>>) -> Result<Cow<'a, BStr>
                 use std::os::unix::ffi::OsStrExt;
                 path.as_os_str().as_bytes().into()
             };
+            #[cfg(target_os = "wasi")]
+            let p: &BStr = {
+                use std::os::wasi::ffi::OsStrExt;
+                path.as_os_str().as_bytes().into()
+            };
             #[cfg(not(unix))]
             let p: &BStr = path.to_str().ok_or(Utf8Error)?.as_bytes().into();
             p
@@ -75,6 +85,11 @@ pub fn try_from_byte_slice(input: &[u8]) -> Result<&Path, Utf8Error> {
         use std::os::unix::ffi::OsStrExt;
         OsStr::from_bytes(input).as_ref()
     };
+    #[cfg(target_os = "wasi")]
+    let p = {
+        use std::os::wasi::ffi::OsStrExt;
+        OsStr::from_bytes(input).as_ref()
+    };
     #[cfg(not(unix))]
     let p = Path::new(std::str::from_utf8(input).map_err(|_| Utf8Error)?);
     Ok(p)
@@ -102,6 +117,11 @@ pub fn try_from_bstring(input: impl Into<BString>) -> Result<PathBuf, Utf8Error>
         use std::os::unix::ffi::OsStringExt;
         std::ffi::OsString::from_vec(input.into()).into()
     };
+    #[cfg(target_os = "wasi")]
+    let p = {
+        use std::os::wasi::ffi::OsStringExt;
+        std::ffi::OsString::from_vec(input.into()).into()
+    };
     #[cfg(not(unix))]
     let p = {
         use bstr::ByteVec;