Move prime decomposition functionality.

Old location of the functionality was in the `prime` directory, new
location is the `decomp` directory. The reason for this move is to
accommodate (future) increased decomposition functionality such as prime
decomposition sieving.
Use int instead of uint64 in decomp functions.
Add prime decomp sieve.
Add SumDivisors function to Decomp.
Add solution 21.
Merge branch 'PE-21'