Demangle using fixed-length disambiguators

[teor2345]

Most UI tests expect a 16-character disambiguator, but rustc-demangle omits leading zeroes.
This can cause test failures on ~1/16 of the changes to demangling tests.

For example: rust-lang/rust#160050 (comment)

This PR adds a Disambiguator wrapper type, so it is impossible to format it incorrectly.

I used https://www.dcode.fr/base62-encoding to generate the encoded examples, you need to select the second alphabet (lowercase first).

[bjorn3]

write!(out, "{:016x}", self.0) would work too, right?

[teor2345]

Unfortunately this is core-only code:

rustc-demangle/src/v0.rs

Lines 5 to 12 in f36e298

	macro_rules! write {
	($($ignored:tt)*) => {
	compile_error!(
	"use `self.print(value)` or `fmt::Trait::fmt(&value, self.out)`, \
	instead of `write!(self.out, \"{...}\", value)`"
	)
	};
	}

[bjorn3]

write!() is in core. I don't know if the reason given for avoiding write!() in #50 is still fully valid given that we had a change to the fmt implementation since:

The second commit is mostly stylistic, trying to enforce a pattern of "not using write! in demangling logic", and only using fmt::Trait::fmt(&value, self.out) - which I'm guessing we do want (for performance reasons?).

In any case I guess the current code is fine.