mbedcrypto: Make bignum_bn2dec() return char*.
Noticed while developing bignum self-checks (upcoming merge request).
This aligns it with libgcrypt/OpenSSL backends which uses char*. It also aligns mbedcrypto's bignum_bn2hex() to use an unsigned cast just like OpenSSL backend.
Checklist
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Commits have Signed-off-by:with name/author being identical to the commit author -
Code modified for feature -
Test suite updated with functionality tests -
Test suite updated with negative tests -
Documentation updated
Reviewer's checklist:
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Any issues marked for closing are addressed -
There is a test suite reasonably covering new functionality or modifications -
Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTING.md -
This feature/change has adequate documentation added -
No obvious mistakes in the code