Correct behavior for reading memory on linux

[JJendryka]

Implements #50

• Use ptrace as a fallback for process_vm_readv
• Handle cross page read correctly
• Fix tests so that ptrace can be used inside them
• Restructure the code to be more readable

[JJendryka]

As far as I know /proc/<pid>/maps is always sorted by address.

I've also thought so, however I cannot find any resource that states so.

[nbaksalyar]

Thank you, this looks great! I left a small comment about the use of sort_by for memory maps.