Demangle C++-style mangling to "Nim style"

[arnetheduck]

Right now, the demangler works by reusing the C++ demangler which in turn means that the syntax in call stacks comes out looking like C++.

For the call stacks produced by this code, we could instead demangle to "Nim style" - ie instead of module::func(ref<type>) we'd have module.func(ref type).

This demangling could be activated by looking at the source code suffix of the line being demangled - if nim, used nim and otherwise use upstream demangling.

The "nim style" of mangling is defined here: nim-lang/Nim#23302