defl_pseudo

Add the 'DEFL' pseudo-operation, which allows re-assigning statement
labels during assembly. Note that this will need a FIXME when binary
code is emitted, since the test case changes the origin several times,
necessitating sorting the statement addresses.

Separated the equates from statement (symbol) labels from the defined
labels via DEFL.

Found and fixed a parser bug when collecting a comment after an
expression. Also simplified the way that expressions are parsed.

A single '$' will return the current assembler program counter's value.