mbedtls: Don't unnecessarily grow the result of a hardware bignum operation

mbedtls: Don't unnecessarily grow the result of a hardware bignum operation

Avoids growing the result of hardware bignum operations
(particularly for multiplication)

Fixes bugs where some Elliptic Curve operations fail or corrupt memory,
as they assume length of the number is never greater than the number of
non-zero limbs.

Includes some general refactoring to standardize terminology.

Closes https://github.com/espressif/esp-idf/issues/1556

Fixes TW12984

Adds test cases for both these issues.