Add support for different types in binary operations.

Well, actually the support is only for mixed ints and floats,
but it works.
Bump to version 0.15.3
Reparaphrase (?) the error messages on wrong types in binary/unary operations.
Use ints in binary ops when possible.

So for example int 2 + float 2.0 returns int 2.
Start some work on scoping.
Add the "include"ing ability.