Improvements to 6502 emulation

[mzattera]

• Math and bitwise functions use a work area that is cleared at each call. It is to be checked how many cells are actually used in calls and clear only the bare minimum that is needed; do it with BF code directly -> Hint isolate the brainfuck code for each function in the C code, replace acces to m[] with functions to track which cell is accessed. -> replace clean_tape with required [-]>

• Check performance of functions and use them accordingly; is mod X 255 faster than AND X 255? -Should we add a SHL call?

[mzattera]

First bullet point above is fixed in v.2.1.