Use pointMult instead of arithmetic.privtopub

pointMult is faster than the pure python arithmetic.privtopub

Additionally in makeCryptor the call to a.privtopub could have just simply be changed to call the local privToPub but then privkey would have been dehexified twice (once in makeCryptor, then again in privToPub) and privToPub would have hexified its result only for makeCryptor to immediately dehexify it. This sort of unnecessary hexifying/dehexifying seems to occur throughout PyBitmessage.