a levenshtein implementation in nqp
when a parent class doesn't exist, throw X::Inheritance::UnknownParent
when a parent class isn't known, search through all blocks (WIP)
these hacks are no longer needed
this code caused strange exceptions:

Method 'ast' not found for invocant of class 'String'
some ineffective faffing about.
fix the rethrow and message. thanks jnthn++
reinstate the give-up-early code.
whitespace fixes for UnknownParent exception.
new threshold logic for levenshtein

it's complicated, so it must be good, right?
changing the case of a letter costs 0.5
switching two adjacent letters costs 1 instead of 2.
factor out $achar, $bchar
hyphens and underscores are cheap.
skip the name of the class itself.
quote all the symbols in UnknownParent exception.
fix the seen array
the lower bound code is flawed in some way.
add an empty line after the suggestions.
changing sigils is cheap, too.