make or/c and contract-stronger? collaborate

in order to make contract-stronger? work better when given the result of or/c
Fix a test error.
Fix contract syntax in unit signature.
Allow TR contracts to be turned on with an env variable.
fix contract-random-generate so it picks the generation method
randomly during generation, not randomly up-front, before
generation
clean up and export support for exercising values with contracts
document contract-exercise
Add a syntax for some asymmetric predicate filters

Relevant for PR 14423
Add docs for TR's filter syntax
Fix check-subforms and reenable TR contracts.
add `custodian-tidy-all`
racket/port: add a flushing tidy callback for `reencode-output-port`

This change solves a long-standing problem that reencoded output
was not flushed on exit, especially in the case that the
current output port is reencoded.
fix `vector-map!` to not mutate immutable vectors
Fix registration of Opaque type alias in REPL

Closes PR 14487
Fix `evaluator.rkt` to work with `dynamic-require`.
Add test that runs TR unit tests with contracts on.
Fix contract profiler path shortening.

Was not preserving list order while shortening.
cleanup for size
Fixing PR14478
Increase timeout.