[Chapel Merge] Adjustments to the visibility of bigint's implemen

Branch: refs/heads/main
Revision: 710a5f5
Author: lydia-duncan
Link: Unavailable
Log Message:

Merge pull request #19281 from lydia-duncan/mpzStruct

Adjustments to the visibility of bigint's implementation details
[reviewed by @bmcdonald3]

Stops documenting the mpz field for the bigint type in the BigInteger
module, and deprecates the mpzStruct method in favor of a new method
called getImpl, which is documented as an implementation detail that may
change in the future.

Resolves #17694

Updates some uses of the now deprecated method to use the new method
instead. Updates some documentation links to the mpz field to not be
links any more - the field still exists, so it's okay for users to refer to it though
it may become private later when private fields are implemented.

Adds a new test locking in the deprecation message for mpzStruct.

Passed a full paratest with futures

Modified Files:
A test/deprecated/BigInteger/deprecateMpzStruct.chpl

A test/deprecated/BigInteger/deprecateMpzStruct.good
M modules/standard/BigInteger.chpl

Compare: https://github.com/chapel-lang/chapel/compare/8a320ba8dc47...710a5f557132