bug#11657: 24.1.50; Byte compiler is a lot slower now, when run interpreted

[Stefan Monnier]

> Another would be to add some instrumentation code to the `pcase' macro,
> to output a message everytime the cache misses.  E.g. use the patch below.

I tried that and found that the `pcase-let' macro was not memoized (it
goes through `pcase' but pcase's memoization fails in that case).
I added explicit memoization to pcase-let, which should
help significantly.

Please try it and tell me if it was the only source of slowdown.

BTW, regarding those problems, I'm experimenting with a change that
calls macroexpand-all directly from readevalloop, so as to avoid such
repeated macro-expansion.


        Stefan