Add an optional --debug-mozjs argument to `mach build` that enables a non-optimized, debug build of mozjs and rust-mozjs. Update the Cargo snapshot to enable new feature support.
auto merge of #3803 : jdm/servo/mozjsdebug, r=mbrubeck

...n-optimized, debug build of mozjs and rust-mozjs. Update the Cargo snapshot to enable new feature support.