Move frontend to frontends/lean

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Fix annoying problem when an integer occurs in the end of a command. Example 'Show 1.' was being parsed as 'Show 1.0'.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add set options to lean_parser. Add support for disabling unicode output. Use channels in lean_parser.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add comments for unicode symbols, fix a typo
Doxygen: exclude src/tests directory
Fix interval<T>::mul to suppress clang warnings
Add SMT frontend (skeletal)