internal/radix51: implement (*FieldElement).Mul32

[FiloSottile]

This pure Go implementation of Mul32 is more than twice as fast as the assembly Mul implementation, and four times faster than the pure Go Mul.

Mul32          7.91ns ± 1%
Mul            18.6ns ± 1%
Mul [purego]   33.4ns ± 0%

Before Go 1.13, where we can't use math/bits because the fallbacks might not be constant time, Mul32 is a little slower, but not nearly as much as the pure Go Mul.

Mul32          9.74ns ± 0%
Mul [purego]   75.4ns ± 1%

/cc @hdevalence

[FiloSottile]

Updated the go.mod version and removed the clever binary syntax.