Added memory tests, improve sym errors

[mnemonikr]

• Introduced memory unit tests.
• Added source for sym::ConcretizationError
• Improved implementation of mem::read_bytes

[mnemonikr]

Coverage for this run:

Filename                      Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
src/mem.rs                         90                32    64.44%          24                 8    66.67%         202                61    69.80%           0                 0         -

Previous:

Filename                      Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
src/mem.rs                         75                29    61.33%          17                 7    58.82%         138                49    64.49%           0                 0         -