parse: Document the grammar
Implement parsing & evaluation of terms

What used to be terms are now just factors.
parse: Terminate barewords at " ' too
parse: Backquote is now used for single-quoting
parse: Introduce ItemList node; allow empty pipeline
Parse and evaluate factor of term list

Note: the evaluation is currently a stub.
eval: Evaluate terms as Cartesian products of factors
eval: A little reorg