Make EXPLAIN output for JIT compilation more dense.

Make EXPLAIN output for JIT compilation more dense.

A discussion about also reporting JIT compilation overhead on workers
brought unhappiness with the verbosity of the current explain format
to light.  Make the text format more dense, and restructure the
structured output to mirror that more closely.

As we're re-jiggering the output format anyway: The denser format
allows us to report all flags for JIT compilation (now also reporting
PGJIT_EXPR and PGJIT_DEFORM), and report the total time in addition to
the individual times.

Per complaint from Tom Lane.

Author: Andres Freund
Discussion: https://postgr.es/m/27812.1537221015@sss.pgh.pa.us
Backpatch: 11-, where JIT compilation was introduced