Implement a Protocol-Oriented Mangler

[CodaFi]

What's in this pull request?

Adds the silt-demangle subtool and three new manglers:

• The GIRMangler now has a more formal mangling scheme defined
• The Demangler can be used to build a mangling node-tree from a mangled name
• The Remangler reprints a demangled node tree into a mangled name

Like any big change, this entailed a lot of refactoring.

Related to #104

[CodaFi]

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