This reworks the "a=" detection to be simpler.
If we detect a variable assignment that produces an error,
simply consume it.
We also take the opportunity to not highlight it as an error,
and add some tests.
Original commit is 1ca05d32d3.
Typing that command in an interactive prompt would make the highlighter thread
eat up CPU and memory. Probably not the right fix; I think the token should
already have been consumed when the error is detected, then there is no need
to consume it when unwinding.
This is the first commit of a series intended to replace the existing
"parse tree" machinery. It adds a new abstract syntax tree and uses a more
normal recursive descent parser.
Initially there are no users of the new ast. The following commits will
replace parse_tree -> ast for all usages.
