bug#46988: 28.0.50; Documenting and verifying assumptions about C code not calling quit or GCing

[Pip Cet]

This is a request for a debugging feature which would verify (at run
time or at compile time or both) that functions we know shouldn't call
GC don't call GC.

See https://lists.gnu.org/archive/html/emacs-devel/2021-03/msg00306.html

Proof-of-concept patch for a runtime check will be attached once this
has a bug number.