Move rounduint64 into mastercore_convert.h/.cpp
Add tests for rounduint64

Some fail!
Change rounduint64, pass tests
Move swapByteOrderN into mastercore_convert.h/.cpp
Move PUSH_BACK_BYTES makros
Test swapByteOrder
Merge pull request #183 from dexX7/mcore-add-convert-test-round

Move rounduint64, swapByteOrderN, test and "fix" rounduint64