fix mangling of booleans and zeros

[simonbyrne]

Fixes #609.

I also noticed that zeros were not being mangled correctly (the n prefix should only be added if the value is negative, not if equal to zero).

[maleadt]

Thanks! We probably should move this to a separate package at some point, and include some better testing (checking against a c++filt JLL or so). For now, good to have a fix.

[simonbyrne]

Another improvement might be to use module names as namespaces.