Myrial UDAs

This PR adds UDAs to Myrial.  JSON compilation is not implemented.  Also, there is no support for combine/merge.

This change was made by sed during a renaming pass.  So, there isn't anything deep going on here.  It just so happens that decomposable aggregates are synonymous with builtin aggregates.

I agree it makes sense to talk about combiners for UDAs.  There are some issues to tackle here.  For example, the combiners for built-in aggregates are expressed in terms of other aggregate functions (e.g., average uses sum and count).  The semantics are tricky enough that I punted for now.