Implement tr/// and class StrDistance.

StrDistance takes the untransliterated and transliterated Strings as
constructor arguments and implements .Bool, .Int and .Numeric. Numeric
defers to .Int, which calculates the replaced characters on invocation.
Bool returns True if any transliteration has occured, False otherwise.
Merge branch 'implement_tr' of github.com:peschwa/rakudo into nom