Added support for arithmetic operations, function definitions, and
primitive bodies in grammar.

Signed-off-by: Jakob Brandsborg Ehmsen <jakobehmsen.github@gmail.com>
Added opcodes, evaluation logic, and compilation logic for arithmetic
operations.
Also started work on function definitions.

Signed-off-by: Jakob Brandsborg Ehmsen <jakobehmsen.github@gmail.com>