Implemented compile time constant folding

eel_m_op() and eel_m_cast() will now check for constant operands, and
whenever possible, perform the operations and return constants, instead
of code issuing manipulators.

Refactored the unary operator implementations out of the VM and into
e_operate.h, where the eel_unop() wrapper was added for constant folding.

Added constfold.eel test; an all constant version of arithmetics.eel.
(Which as of now, is also  mostly copying constants into registers instead
of actually performing calculations at run time.)

Closes #74.