bug#43725: 28.0.50; Include feature/native-comp into master

[Andrea Corallo]

Hi all,

FYI today I did some work to put the testsuite in shape also for the
native build (vanilla pass clean already).

The main challenge is related to the fact that the testsuite does large
use of primitive redefinition (tipically through `cl-letf').

As redefining primitives does not take effect in optimized code I
defined a macro (`advice-flet') with similar use but to advice instead
and put it in place in a numer of tests.  You'll see this work in
d07d7ab1a0 825e85b393.

I hope this approach is accettable (thought was good to ask for a
feedback), otherwise we can revert and find another solution.

ATM the testsuite for the native build runs still not clean, I'll finish
with cleaning it up.

  Andrea