glib/GStringPtr: Add as_str() and Deref<Target=&str>

I was updating some code to glib 0.18 and we were previously
using `as_str()` - which is what the default Rust nomenclature
for an *infallible* reference-to-reference conversion uses.

The `to_str()` is used for fallible conversions
(e.g. https://doc.rust-lang.org/std/ffi/struct.OsStr.html#method.to_str)
which this is not.

glib/src/gstring.rs

@@ -694,10 +694,17 @@ impl GStringPtr {
     // rustdoc-stripper-ignore-next
     /// Returns the corresponding [`&str`].
     #[inline]
-    pub fn to_str(&self) -> &str {
+    pub fn as_str(&self) -> &str {
         self.to_gstr().as_str()
     }
 
+    // rustdoc-stripper-ignore-next
+    /// This is just an alias for [`as_str`].
+    #[inline]
+    pub fn to_str(&self) -> &str {
+        self
+    }
+
     // rustdoc-stripper-ignore-next
     /// Returns the string's C pointer.
     #[inline]
@@ -724,6 +731,15 @@ impl Clone for GStringPtr {
     }
 }
 
+impl Deref for GStringPtr {
+    type Target = str;
+
+    #[inline]
+    fn deref(&self) -> &str {
+        self.as_str()
+    }
+}
+
 impl IntoGlibPtr<*mut c_char> for GStringPtr {
     #[inline]
     unsafe fn into_glib_ptr(self) -> *mut c_char {