raco exe: enable on-demand parsing of embedded bytecode

Creating a stand-alone executable could slow down a program,
because bytecode embedded in an executable was not treated
in the same way as bytecode loaded from files. The difference
was in on-demand parsing of bytecode --- and now it's enabled
for embedded bytecode, too.
fix `raco exe' test
Adding customization of command-char to web-server/template

This entails adding it to include/text and read-syntax-inside as well.
fixed (~r 0 #:notation 'exponential ....) bugs
some random testing for (~r #:notation 'exponential ....)
fix estimate of order of magnitude for ~r
Remove contract on unit-exported definition.
Add contract for seen parameter.
Refactor subtype.rkt not to use exceptions.

Matthew suggests that using exceptions for failure
in the implementation of `subtype` is a performance
problem.  This commit removes all use of exceptions
for failure in subtype.rkt, replacing it with the
standard Racket #f/value option.

Extensive use is made of the `subtype-seq` form,
which is basically do-notation for the subtyping
monad.
add missing guard

related to PR 13741
tighten up the contract for get-display-size and get-display-left-top-inset
Forgot to commit example
Cleanup tc-lambda-unit.
New Racket version 5.3.4.10.
Some style cleanups.

Mainly reogranize the code + exports + docs of the additional list
convenience functions.

Also, add a custom printer to a "running" struct in
`racket/private/promise', so there's a sensible output if the value
happens to leak outside of a promise (eg, when debugging).
Add `permutations' and `in-permutations'.
Temporary hack: avoid `permutations' clash until its renamed in `math'.

(Should undo this when the rename is done.)
Make the stop value for `in-producer' optional.

There are many cases where you just want to use some other tool like
`#:break' to stop the iteration, so no need to make up a bogus stop
value and no need to spend time checking it.
Fix inclusion of "keep-dirs.rktd".

The file should be considered as source even though it's in the "doc"
directory.
Combine the two zo-size tests into one.

Also beautify the output.