When two identifiers follow eachother it could be two instruction
mnemonics or one instruction mnemonic and one operand. To fix this
TOKEN_NEWLINE has been reintroduced as a semantic token. The grammar has
been changed to allow empty statements and every instruction and
directive has to end in a newline. Labels do not have to end in a
newline.
In addition to updating the grammar, the implementation of tokenlist,
ast and parser has been updated to reflect these changes.
