Earley Parser Improvements

[srhickma]

Re-Implemented the Earley parser to fix issues seen with epsilon productions in #11.
The resulting parser is different in three ways:

• epsilon productions are predicted using the nullable symbols of a grammar, rather than using and extra completion loop.
• the parse tree is produced by recursing through a modified earley item chart (parse_chart) using a depth-first search to find only the first parse tree. This algorithm replaces an algorithm which recursively built the entire parse forest using "back pointers" established during the recognition phase.
• the warning for ambiguous parses was removed after it was discovered that grammar and scan producing two parse trees did not trigger the warning (false negative).

As a result of these modifications, the performance of the parser has drastically improved, and memory usage during parse-tree construction has been reduced (since we only build a single parse tree rather than the whole forest).