change stlc-sub's 'check' function to not assume
its input is well-typed
add missing value production

also, use redex-check? instead of redex-check in 2 places
fix subst function

- its contract said it accepted M's as the third argument, but it
  worked only for v's
- remove two redundant cases
new slate of stlc-sub bugs
redex: fix rvm ordered generation in n benchmark
adjust enum distributions
redex: update virtual machine benchmark diffs
Handle init/init-field order correctly in TR

Also handles the case where init and init-field clauses
are interleaved, in which case they need to be interleaved
in the type.

Closes PR 14408
Simplify internal data for TR classes with kws

Use keywords instead of literals for communicating
internal information from the TR front-end macro to the
back-end.
Remove unused imports and a useless comment
add tests to the poly-stlc model
reapply the diffs to get the new p values set for enumeration-based random generation
add + to poly-stlc
adjust check function for poly-stlc and put Jay's counter
example for bug #2 into the file
rename make-mutants because these aren't mutants in the usual sense of the testing literature
don't refresh generator on timeouts to avoid looping timeouts
exception handling for vm model
add exit message with counts
Add convenience enumerator approximation function
unbreak after rename of make-mutants.rkt