Add support for custom demanglers

[DaniPopes]

Adds support for custom demangler functions with the signature fn(&str, &mut Buffer) -> std::fmt::Result where Buffer is an opaque type that only exposes an implementation of std::fmt::Write.

The default implementation is moved to a standalone function, and the buffer is now cleared before any demangling is done as tracy guarantees:

When a call to ___tracy_demangle is made, previous contents of the string memory do not need to be preserved.

Follow-up to https://github.com/nagisa/rust_tracy_client/pull/102.