Move debug functions to Window IDL.

This is a cleaner way to expose those functions, and makes it possible to
remove a significant amount code in rust-mozjs.

The assert() function is no longer exposed, as it was unused and not very
useful.
auto merge of #2381 : Ms2ger/servo/mozjs-spring-cleanup, r=jdm

This is a cleaner way to expose those functions, and makes it possible to
remove a significant amount code in rust-mozjs.

The assert() function is no longer exposed, as it was unused and not very
useful.