Memory usage improvements

[SparkiDev]

Description

kdf.c: wc_PRF() - No need for previous, reuse current. sha256.c: Transform_Sha256() - Add slow but small version for many register implementation. sp_int.h: Change 'used' and 'size' fields to 16-bit types when possible. sp_int.c: Fixes for 16-bit used.

Testing

Memusage testing

Checklist

• [ ] added tests
• [ ] updated/added doxygen
• [ ] updated appropriate READMEs
• [ ] Updated manual and documentation