check?: a minor efficiency/debug clarity improvement

I have discovered that the nondeterminism introduced by MDO
clutters the logs.
parse-sequence*: fix position table not being returned in all the right cases
Provide a separate debug system, dependent on CL-CONTAINERS

  o  line/column numbering, character peeking
basic.lisp: add some docstrings
Define C?, an ergonomic shortcut for CHECK?

..ergonomics do matter during wild debug sessions..
Add MDO*: like NAMED-SEQ*, but with MDO syntax: the last element must be a parser.
Add BEFORE*: find a p before q, doesn't consume q.
Add CHOOKAHEAD?: return result if p matches, but do no advance