Demangle symbols names in fzf

[cgaebel]

When showing the user an fzf to select a trigger symbol, show demangled symbols instead of mangled ones.

Symbols specified at the command line, I think, should still be mangled to ease copy+paste between apps and to avoid forcing us to write a name mangler too.

The message we show after a user selects a symbol should print its mangled name so the user can copy+paste it into future magic-trace invocations.

[hlian]

@Clyvegassant is going to take this

[hlian]

@ClyveGassant is going to work on Rust demangling next