Bignum: extract core_sub_int from the prototype

[tom-cosgrove-arm]

Description

Extract MPI_CORE(sub_int) from the prototype. Fixes #6231

Status

READY

Requires Backporting

NO

Todos

• [x] The MPI_CORE(sub_int) and its dependencies are taken from the prototype and credit is given to Hanno
• [x] New macros are expanded/removed
• [x] The legacy functions in bignum.c are calling the new function wherever possible
• [x] Extensive unit tests are added for all new functions

[tom-cosgrove-arm]

Based on #6083 like the others. Needs unit tests and possibly using in the existing code

[tom-cosgrove-arm]

Now ready for review. This is based on top of the montmul PR #6083. The actual work in this PR is the last 4 commits.

[tom-cosgrove-arm]

Rebased on top of the montmul PR 6083, which has had a doc comment fix applied. Still the actual work in this PR is only the last 4 commits.

[tom-cosgrove-arm]

Rebased on top of the latest montmul PR 6083. The actual work in this PR is only the last 4 commits.

[tom-cosgrove-arm]

Oops, forgot that we're removing the mbedtls_ prefix from the test case names, so added another commit to do that.

This PR is based on the latest montmul PR 6083. The actual work in this PR is only the last 5 commits.

[gilles-peskine-arm]

Since https://github.com/Mbed-TLS/mbedtls/pull/6083/commits is merged, please rebase.

[tom-cosgrove-arm]

Now ready for (re-)review; uses the updated python test framework