Added simple 'if' statement support and restructured the implementation.

= mylang extensions =
* Added simple 'if' statement support.
  - No 'else', etc. clause support.
* Added logical expression support:
  - 'True' and 'False' constants,
  - '&&' (conjunction) and '||' (disjunction) operators,
  - '==' (equals) and '!=' (not equals) boolean comparison operators.

= mylang implementation details =
* Got rid of the finite state machine for parsing const tokens (like '+', '&&', etc.)
in the
lexer.
  - From now on, to support a new const token in the lexer, you just have to modify the
__const_toks dictionary defined in the lexer constructor.
* Renamed lexer tokens and parser nodes in a more syntax-wise fashion. The change is very
questionable, of course. Let's see if it fits.
* Restuctured the parser heavily to make it more concise.
  - The __try_parse_token and __parse_token parsing support methods were added.
  - The existing parsing methods were adjusted with the respect to the new parsing support
methods.