Auto merge of #568 - Redfire75369:feature/external-string, r=jdm
Auto merge of #568 - Redfire75369:feature/external-string, r=jdm

Fixed Signature of External String Callbacks Trap

Yet another mistake on my part that I didn't catch. `wchar_t` is platform-dependent and definitely not `char16_t`.
In fairness, they're the same on windows which confused me.

servo/rust-mozjs,servo/rust-mozjs,servo/rust-mozjs,servo/rust-mozjs