Implement rejecting use of private fields

[gilles-peskine-arm]

Implement the strategy for rejecting the use of private fields investigated in #4372 to the private fields of the classic API in 3.0 documented in #4371, as well as to PSA API private fields in both 2.2x and 3.0.

[mpg]

Note: we have about a hundred structures in public headers. We probably don't need to do all of them at once, though.

[gilles-peskine-arm]

We probably don't need to do all of them at once, though.

It'll be easier to do them all at once with a global search-and-replace.

[mpg]

I'm probably missing something, but I was thinking it would need to be done struct by struct.

[gilles-peskine-arm]

This was done in #4511