Rename plus/minus/mult to add/sub/mul

[blaxill]

Perform the changes in issue #132 :

• sat{Plus, Min, Mult} to sat{Add, Sub, Mul}
• bounded{Plus, Min, Mult} to bounded{Add, Sub, Mul}
• ExtendingNum plus, minus, times to add, sub, mul

This passes clash-prelude unittests and doctests, but appropriate changes also need to be made to clash-compiler, clash-lib and clash-ghc.

[blaxill]

Without the ExtendingNum changes, the required changes to clash-compiler, clash-lib and clash-ghc would be less severe, but I think this PR would be the most consistent naming scheme.

[blaxill]

I've reverted the primitive name changes (plus#, minus#, times#)

[christiaanb]

Thanks!