[SSP16ASM] Implement a two-pass assembly process

[jdesiloniz]

After introducing the missing instruction LD A, addr (as in LD A, 050 to access mem bank 0 - addr 50), the whole labeling system has broke (in 0.1.3). The whole labeling/constants system is too flimsy and convoluted. This would be fixed by redoing the whole assembly process with a proper two-pass approach:

1. parse code in a first pass to calculate label locations based on each instruction size (properly calculated, not as of now).
2. replace labels and constants with the actual values
3. perform second pass with the instructions (along with proper values) to generate assembled file.