Also, translate the README to Markdown. And remove xcvb support.

Try to avoid stack overflow, issue #1.

Indeed, SBCL's new FORM-TRACKING-STREAM doesn't provide FILE-POSITION,
and we can make do with a limited form of infinite-lookahead where we
can flush the lookahead buffer for sure before we functions that don't go
through our buffer.

the scribble-at test.

Tweak the readtable API, to be in synch with fare-quasiquote.

Some more error checking.