Refactoring upon hands on

• All solutions have been reworked so that declarations and definitions are separated.
• Two optional exercises that were problematic have been removed (#125 (closed))
• 3 new exercises have been added (one is the reworking of one deleted in #125 (closed))

@jediaz I am not sure I have fixed all remarks stemming from previous MRs; I'll check that tomorrow.

@vrouvrea I put @jediaz as reviewer but feel free to chime in; in peculiar I would like to make sure you're ok with new exercises (34, 38 and 43).

Closes #124 (closed), #125 (closed)