refactor: improve widestring conversion utils

- Don't use `WString::from_str` for `str`s which are available at
  compile-time. Use `L!(input).to_owned()` instead. The reason for this
  is that `WString::from_str` can cause problems if the input contains
  PUA bytes which we use for our custom encoding scheme. In such cases,
  `bytes2wcstring` should be used, to avoid problems when decoding the
  `WString`. Removing harmless usages of `WString::from_str` allows us
  to focus on the potentially dangerous ones which don't convert
  `str`'s that are compiled into the binary.
- Make `cstr2wcstring` actually take `CStr` as its input. The former
  version was only used in one place, and the conversion to `CStr`
  should happen there, where it can be checked that the conversion makes
  sense and is safe. The new version is used in
  `src/env/environmant.rs`, to avoid `to_bytes()` calls cluttering the
  code there.
- Add `osstr2wcstring` function. This function also works for `Path`.
  Now, these types can be converted to widestrings with much less
  syntactic clutter.

src/io.rs

@@ -1,5 +1,5 @@
 use crate::builtins::shared::{STATUS_CMD_ERROR, STATUS_CMD_OK, STATUS_READ_TOO_MUCH};
-use crate::common::{bytes2wcstring, wcs2bytes};
+use crate::common::{bytes2wcstring, str2wcstring, wcs2bytes};
 use crate::fd_monitor::{Callback, FdMonitor, FdMonitorItemId};
 use crate::fds::{
     BorrowedFdFile, PIPE_ERROR, make_autoclose_pipes, make_fd_nonblocking, wopen_cloexec,
@@ -725,7 +725,7 @@ pub fn append_char(&mut self, c: char) -> bool {
     }
 
     pub fn append_narrow(&mut self, s: &str) -> bool {
-        self.append(&bytes2wcstring(s.as_bytes()))
+        self.append(&str2wcstring(s))
     }
     // Append data from a narrow buffer, widening it.
     pub fn append_narrow_buffer(&mut self, buffer: &SeparatedBuffer) -> bool {