extract and fix unmangling in assembly_filter

[Heiko Behrens]

As part of #4 the assembly_filter() has a chunk of code that unmangles and beautifies labels:
https://github.com/HBehrens/puncover/pull/4/files#diff-5dfd4c18d4ead9f05b1149c16d3e6e93R114

1. This code should be extracted and tested
2. By using a lazily-initalized version of unmangling in renderer.display_name_for_symbol_name it could also resolve symbols that were inlined