Write/read IR to/from disk

by Aliaksei Palkanau for JRuby

This feature will enable JRuby to avoid parsing Ruby source, build AST, and generate IR across runs. This feature might be especially useful for faster bootup of short-running programs (ex: tests).This project would have to ensure that the (human-readable) textual output captures all necessary information from IR so that when it is read back in, the IR can be reconstructed without loss of information.