Implement addition for real literals.
Implement simple decimal real literals.

The following decimal real literals are supported:

    1.2d
    .4d
    123.456e-2d
    123.456E-231d
    3.45e3d
    32.2e+3d
    1.5Dmb
REFACTOR: Real literal Ast building.

Remove duplicated code.
Extract method for building double real literals.
Implement addition for real decimal literals.