v0: replace skip_* methods with print_* methods in a "skip printing" mode.

[eddyb]

My motivation for this change is that I wanted to implement structural constant value demangling (for rust-lang/rust#87194) and having to implement both skip_const and print_const for the new constant values seemed redundant, and also potentially risky (i.e. the two could go out of sync).

FWIW, the C demangler I wrote for libiberty just doesn't have this problem, as it took the "skip printing" route from the start, and I believe other implementations do something similar as well.

While the print_* methods may be less efficient when the out field is None, than the old skip_* implementation, I don't think the performance difference warrants dealing with the maintenance burden of duplication.

Worst case, if performance does become a concern, we can make Printer generic over a type that provides the Option<&mut fmt::Formatter> through a method, which could let LLVM optimize print_* methods to be more or less the same as the old skip_* methods (for Printer<SkipFmt>).

[alexcrichton]

Seems reasonable to me!