Complete sloppy integer multiplications

[albinahlback]

We should have the basecases for these integer multiplication types in assembly:

• mullo
• mulhi
• sqrlo
• sqrhi

Currently only parts of these are implemented. (mulmid could also be implemented, but probably just for very specific forms.)

[fredrik-johansson]

About mulmid: the general four-parameter version would be very messy to do with hardcoded basecases, but the schoolbook algorithm could presumably be done in the style of mpn_mul_basecase.

David Harvey's mpn_mulmid in GMP is a restricted version depending only on a single parameter n, useful for pure balanced middle products. This one could use hardcoded basecases. However, we would need to port the whole Karatsuba machinery from GMP along with assembly routines for linear operations to benefit from these basecases for larger n.