Friendlier optimization log

[sampsyo]

There are a few relatively easy plumbing improvements that would make the ACCEPT log much easier use.

• Split the log messages by source file. This is sort of a consolation prize for not offering separate compilation in ACCEPT, which I believe is hard/impossible: we should at least split out the messages for a given source file so they're not intermixed; this way, you have some shot at finding the message you want.
• Sort the messages by line number across all optimizations. Currently, the loops (for example) are ordered willy-nilly, which is incredibly distracting.

This would require adding a new logging API that buffers a "stanza" of logging messages along with the source location (file and line number). Later, the log would be dumped out all at once after sorting and splitting.

[sampsyo]

Fixed as of 77875de294548170fe39d490652c18a6ebe12dfb.