implemented a memory model based on memory maps.

Highlights:

1. Reimplemented elf parser (will close #7)
   It still uses bitstring and strings, but loads only headers and
   tables.

2. Factored out a memory from image.
   Now memory is very lightweight and represented just as two pointers
   into memory mapped file

3. Added Table module that allows to perform arbitrary memory mapping,
   like mapping memory to machine code, symbols etc.

4. Reimplemented Image module, so that it now uses new Memory and Table
   modules.

As a result, other than new abstractions we got a significant speedup,
readbin now parses executable 5-times faster.

This is a work-in-progress. I still need to add more tests.