Add reader support for string based constants.

The reader now supports reading string and character constants. The
following tests now pass:

Reader.read_string_constant_recognizes_input
Reader.read_string_constant_recognizes_input_with_escape_quote
Reader.read_character_constant_recognizes_input

Again, the fact that these tests pass is not a proper indicator that the
whole functionality is implemented. We still don't have tests for
verifying the correct rejection of invalid input. It seems expected input is
correctly handled, though.