Calc: speed up math-read-preprocess-string (bug#67536)

Calc: speed up math-read-preprocess-string (bug#67536)

`math-read-preprocess-string` is one of the bottlenecks of `calc-eval`
and was unnecessarily slow even with no substitutions made.
This affected org-mode in particular, where `calc-eval` is called
repeatedly to recalculate tables.

Reported by Raffael Stocker who also wrote the unit tests here.

* lisp/calc/calc-aent.el (math--read-preprocess-re-cache): New.
(math-read-preprocess-string):
Use math--read-preprocess-re-cache, first computing it if necessary.
* test/lisp/calc/calc-tests.el (calc-math-read-preprocess-string):
New test.