Fix typo in ast

1-character-deletion.
Merge pull request #1865 from speedmanly/patch-1

Fix typo in ast -- PR by @speedmanly
Compile coro-profiler binaries into a separate build directory.
Merge branch 'next' of github.com:rethinkdb/rethinkdb into next
Merge branch 'next' into sam_alt
Merge branch 'sam_alt' into sam_alt_snap