Add sat::LiteralSequence and its begin and end for range-based for statement.
Change unique check to use std::adjacent_find.
Add sat::VariableSequence and its begin and end for range-based for statement.
sat::LiteralSequencer: Change a constructor to explicit.
Remove an unused header.
Remove sat::LiteralIterator and its functions because its were unused.
Add functions for range-based for statements.