This commit tweaks WebIDL expansion of the "long long" and "unsigned long long"
types to expand to a union of an 32-bit integer and a double. This reflects how
almost none of the APIs on the web today actually work with a `BigInt` (what the
previous Rust type of `i64` translates to) and how JS itself fundamentally
operates with these APIs.
Eventually this may not be necessary if we can natively connect to C++ engines
with the `i64` type, but until that day comes this should provide more useful
interfaces as they shoudl work in all browsers.
Closes#800
b7d7d28418