Bignum: extract core_mul from the prototype

[tom-cosgrove-arm]

Description

Extract MPI_CORE(mul) from the prototype. Fixes #6226

Status

READY

Requires Backporting

NO

Todos

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

[tom-cosgrove-arm]

Unit tests added; this is now ready for review

[tom-cosgrove-arm]

Rebased on top of montmul, which has had review comments applied. Still the actual work in this PR is only the last 3 commits.

[tom-cosgrove-arm]

Rebased on top of the montmul PR https://github.com/Mbed-TLS/mbedtls/pull/6083, which has had a doc comment fix applied. Still the actual work in this PR is only the last 3 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 3 commits.

[tom-cosgrove-arm]

This is now rebased on top of the latest montmul PR 6083. The actual work in this PR is still just the last 3 commits.

[gilles-peskine-arm]

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