Assembler simplification

[gzanitti]

This is a WIP of the assembler simplification.
The main idea is to drop the use of the pending vector that accumulates bytecodes until the definition of a label is reached.

There is still a lot to optimise, but the general idea is as follows:

• Scan the code to identify labels and macros (surely this can be removed).
• Process the code in its entirety, expanding macros where possible and saving label positions as they appear.
• A final pass to concretise all the labels, assigning them their corresponding value.

As I said before, this is still a work in progress, as I don't think it's necessary to do three passes through the code.