Introduce expression initialization hook?

Hi,

It looks we're creeping towards agreement in
http://archives.postgresql.org/message-id/20161206034955.bh33paeralxbtluv%40alap3.anarazel.de

It'd be fairly easy to add a hook in its ExecInstantiateExpr(), which'd
allow extensions to provide newer (presumably faster) ways to evaluate
expressions.

I myself have no use for this, I'm determined to get JITed expression
evaluation into core. But I could see others finding it useful.

Even if the concensus were that we want a hook there, I do not plan to
add it before this is committed, and a few other items are addressed.
But the discussion might take some time, so ...

- Andres