Aperture macros can get complex with arithmetical operations,
variables and variables substitution.
Current pcb-tools code just read each macro block as an independent
unit, this cannot deal with variables that get changed after used.
This patch splits the task in two: first we parse all macro content
and creates a bytecode representation of all operations. This bytecode
representation will be executed when an AD command is issues passing
the required parameters.
Parsing is heavily based on gerbv using a Shunting Yard approach to
math parsing.
Integration with rs274x.py code is not finished as I need to figure out
how to integrate the final macro primitives with the graphical primitives
already in use.
