Ease integration of MAYO on embedded platforms

[mkannwischer]

This patch does some refactoring to allow the direct integration into PQM4. In particular, it does the following changes:

• support API using size_t rather than unsigned long long (behind a flag)
• save stack space in signing and expand_sk (not having multiple copies of esk on the stack)
• fix warnings about shadowing and unused parameters (esp. when using static params)
• naming of AES and SHAKE that is compatible with PQM4
• move sample_solution to bitsliced_arithmetic.c so it's easier to replace in optimized implementations

[mkannwischer]

Thanks @bhess!