Switch from itanium-abi package to demangler package.

[kquick]

See https://github.com/GaloisInc/demangler. The demangler package is both faster and also more complete with regards to demangling coverage (although still not 100%).

[RyanGlScott]

One challenge here is that some parts of crucible-llvm define C++ overrides that match on parts of the itanium-abi type-related AST to determine if the override should apply or not:

crucible/crucible-llvm/src/Lang/Crucible/LLVM/Intrinsics/Libcxx.hs

Lines 280 to 299 in 50d1550

	-- | An override of the @bool@ operator (cast) on the @sentry@ class,
	--
	-- @sentry::operator bool()@
	sentryBoolOverride :: (IsSymInterface sym, HasPtrWidth wptr, wptr ~ ArchWidth arch)
	=> SomeCPPOverride p sym arch
	sentryBoolOverride =
	trueOverride ["basic_ostream", "sentry"] $ \_nm decodedName ->
	case decodedName of
	ABI.Function
	(ABI.NestedName
	[ABI.Const]
	[ ABI.SubstitutionPrefix ABI.SubStdNamespace
	, _
	, ABI.UnqualifiedPrefix (ABI.SourceName "basic_ostream")
	, _
	, ABI.UnqualifiedPrefix (ABI.SourceName "sentry")
	]
	(ABI.OperatorName (ABI.OpCast ABI.BoolType)))
	[ABI.VoidType] -> True
	_ -> False

The demangler package intentionally does not export its demangled AST data types as part of its API, however. As such, it's not obvious to me how to reimplement this code using demangler.