Fixed region merging during memory mapping

[kingofpayne]

Previous implementation of map_space has some issues :

• New region size is miscalculated when an overlap happens, probably due to : https://github.com/Ledger-Donjon/rainbow/blob/1b754455ad1798067bbbeb867dd8ec975ae04637/rainbow/rainbow.py#L166
• Algorithm fails if the new region completely includes an already mapped region (this case is not handled)
• Algorithm fails if more than 1 region overlap

Those issues have been found by trying to execute the pin_fault.py example, which fails on my current configuration.

The changes propose a new implementation for the region merging algorithm. It must be verified and tested.