…o generate all variants with the new mode generate-variants

…ass for the moment.

- Adding comments in a separate line to avoid wrapping.
- Providing an API to the user of FlowTree class to generate variants.
- Using const_iterators instead of index in FlowTree class.
- Using a struct to represents the conditional block as a unit inside FlowTree.

… variants_.

- Listed the macros as they appear in conditionals.
- Gave each of them a unique ID and stored them in a map.
- Used a more detailed if_blocks struct to help tracking edges.
- Used bitset in DFS to know for each variant which macros are defined.

- Removed variants counter from the class members.
- Added a new struct called Variant that contains all the vairant's data:
  - Its TokenSequence.
  - bitset that shows if the i-th macro is visited/assumed or not.
  - bitset that shows if the i-th macro is defined or not.
- Modified the VariantReceiver to accepts the new struct Variant,
  instead of just TokenSequence.
- GenerateControlFlowTree() is now a private function, such that
  GenerateVariants() is the only API the user needs.
- Added a using declaration for std::bitset<128>.

- Added API GetUsedMacros that returns all the used macros in conditionals.
- Changed "assumed" bitset name to "visited".
- Added description to "ConditionalBlock".
- Added more testing using closures.
Preprocessor tool:
- Kept the subcommand part.
- Available subcommands for now "strip-comments", "multiple-cu",
  and "generate-variants".

- Moved ConditionalBlock to private section in FlowTree class.
- Removed unwanted debug code from the preprocessor tool.
- Fixed output streams issues in the preprocessor tool.