Use C string literals & deprecate `c_str!`

Redo of !624 (closed) based on comments.

Besides C string literals, I fixed a few trivial Clippy lints that don't affect anything.

I also found a test that doesn't compile (unrelated to my changes), but I just marked it as a fix-me for later.

Draft while I recompile everything overnight and test tomorrow to ensure nothing broke.

src/c_str.rs

@@ -54,7 +54,7 @@ impl<'a> CStr<'a> {
     pub const fn as_ptr(self) -> *const c_char {
         self.ptr.as_ptr()
     }
-    pub unsafe fn from_bytes_with_nul_unchecked(bytes: &'a [u8]) -> Self {
+    pub const unsafe fn from_bytes_with_nul_unchecked(bytes: &'a [u8]) -> Self {
         Self::from_ptr(bytes.as_ptr().cast())
     }
     pub fn from_bytes_with_nul(bytes: &'a [u8]) -> Result<Self, FromBytesWithNulError> {