Add a 'parse from string' option for fuzzy java

Summary:
This is handy when trying to match bigger ASTs; this way
the source can include a java string, parse it to fuzzy
ast, abstract the positions, and then use `=` to compare
two trees.

Test Plan: doitlive

Reviewers: pad

Reviewed By: pad

Differential Revision: https://phabricator.fb.com/D905322