Expose function to obtain a PropertyKey from a JSString. (#578)

* Expose function to obtain a PropertyKey from a JSString.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Add doc link.

Co-authored-by: sagudev <16504129+sagudev@users.noreply.github.com>
Signed-off-by: Josh Matthews <josh@joshmatthews.net>

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>
Co-authored-by: sagudev <16504129+sagudev@users.noreply.github.com>

Author: jdm

mozjs-sys/src/jsid.rs

@@ -44,6 +44,14 @@ pub fn SymbolId(symbol: *mut Symbol) -> jsid {
     AsPropertyKey((symbol as usize) | (PropertyKeyTag::Symbol as usize))
 }
 
+#[inline(always)]
+/// <https://searchfox.org/mozilla-central/rev/1f65969e57c757146e3e548614b49d3a4168eeb8/js/public/Id.h#183>
+pub fn StringId(s: *mut JSString) -> jsid {
+    assert!(!s.is_null());
+    assert_eq!((s as usize) & JSID_TYPE_MASK, 0);
+    AsPropertyKey((s as usize) | (PropertyKeyTag::String as usize))
+}
+
 impl jsid {
     #[inline(always)]
     fn asBits(&self) -> usize {