This commit adds support for passing `&mut FnMut(..)` to JS via imports. These
closures cannot be invoked recursively in JS (they invalidate themselves while
they're being invoked) and otherwise work the same as `&Fn(..)` closures.
Closes#123
Nowadays the compile times are mitigated with incremental compilation and
otherwise it's much more ergonomic to run only one test if they're all in the
same suite.
