Levenshtein distance column HINT

Add a new HINT -- a guess as to what column the user might have intended
to reference, to be shown in various contexts where an
ERRCODE_UNDEFINED_COLUMN error is raised.  The user will see this HINT
when he or she fat-fingers a column reference in an ad-hoc SQL query, or
incorrectly pluralizes or fails to pluralize a column reference.

The HINT suggests a column in the range table with the lowest
Levenshtein distance, or the tied-for-best pair of matching columns in
the event of there being exactly two equally likely candidates (these
may come from multiple RTEs, or the same RTE).  Limiting to two the
number of cases where multiple equally likely suggestions are all
offered at once (i.e.  giving no hint when the number of equally likely
candidates exceeds two) is a measure against suggestions that are of low
quality in an absolute sense.

A further, final measure is taken against suggestions that are of low
absolute quality:  If the distance exceeds a normalized distance
threshold, no suggestion is given.
Add state variable to manage things