Merge FUN_STATIC closure with its SRT

Summary:
The idea here is to save a little code size and some work in the GC,
by collapsing FUN_STATIC closures and their SRTs.

This is (4) in a series; see D4632 for more details.

There's a tradeoff here: more complexity in the compiler in exchange
for a modest code size reduction (probably around 0.5%).

Results:
* GHC binary itself (statically linked) is 1% smaller
* -0.2% binary sizes in nofib (-0.5% module sizes)

Full nofib results comparing D4634 with this: P177 (ignore runtimes,
these aren't stable on my laptop)

Test Plan: validate, nofib

Reviewers: bgamari, niteria, simonpj, erikd

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D4637