Remove the streams extension (just the stub)

Summary: The "stream extension" is actually a subset of the standard extension.

Reviewed By: @fredemmott

Differential Revision: D1561554
Admin command to dump regex cache

Summary: By the end of the weekend some machines have accumulated an oddly
large number of regular expressions.  We can use this to figure out what they
are and fix them.

Reviewed By: @mclarkelauer

Differential Revision: D1563554
Change CONTRIBUTING.md to suggest "probably easy" issues

Reviewed By: @paulbiss

Differential Revision: D1564003