Improve choice of default stack addresses

[yhql]

Current hardcoded values can lead to mapping problems, and changing it manually in a script is abit cumbersome.

Should implement:

• Automatic picking of an address in RAM when defined, that is not in an already-defined section
• provide an api to easily set stack address and size

[protopyte]

I recently did:

  emu["sp"] = 0x20002000
  emu.map_space(emu["sp"]-4096, emu["sp"])

which I wouldn't qualify as "cumbersome".

[ddddavidee]

I think something is implemented here : https://github.com/jevinskie/emusca ( -> https://github.com/jevinskie/emusca/blob/fd838219904bded63e91a09953c3659bcb791b16/emusca.py#L350)