Which of the two types is the source and which is the destination? Can we make the API more obvious to read? How about `Convert.from_to('integer', 'float', i)`?
Other conventions I know are `.to_s` and `.from_xml` but they aren't applicable here because we can have complex types. Or perhaps we should construct shortcuts, like `Convert.float_to_integer`, `f2i`, `int_from_float`...