Change ParseContext#splice() to use absolute offsets
Add a "Group" consumer to help with debugging.
Override ParseException#fillInStackTrace()

Improves performance. The stack of ParseException is seldom useful anyway.
Remove unused Contexts.create() method.
Implement node caching
Modify Verifiers to throw exception

A simple yes/no is not very helpful in
solving verification issues. Made Verifiers
throw a Fail exception with more information
on which verifier and node caused the
failure.