Instead of going left and right, finding operators chars and replacing them
with nulls, we parse expressions in a more orderly manner, one chunk at a
time. I think it qualifies as "recursive descent", but I'm not sure.
This allows us to preserve the string we parse and should also make the
implementation of parens much easier.
This should make tests a bit more convenient to write and debug.
Moreover, begin de de-IX-ization of parseExpr. I have, in a local WIP, a
parseExpr implemented using a recursive descent algo, it passes all tests, but
it unfortunately assembles a faulty zasm. I have to find the expressions that
it doesn't parse properly.
But before I do that, I prefer to commit these significant improvements I've
been making to tests harness in parallel of this development.
