libsysprof: Support demangling rust symbols using the v0 scheme

This doesn't add support for the legacy symbol mangling scheme which is currently the default pending support in tools for the v0 symbol mangling scheme. The legacy symbol mangling scheme is similar enough to C++'s symbol mangling scheme that demangling them using the C++ demangler generally produces readable symbols. The v0 scheme is entirely custom and due to backreferences and encoding all generic arguments not very readable when mangled, so supporting it is more important than supporting the legacy scheme.

Part of #96 (closed)