ENH: improve handling of mismatched brackets, forgotten ';' (issue #762)

- flags the following type of problems:

  * mismatches:

        keyword  mismatch ( set of { brackets ) in the } entry;

  * underflow (too many closing brackets:

        keyword  too many ( set of ) brackets ) in ) entry;

- a missing semi-colon

        dict
        {
           keyword  entry with missing semi-colon
        }

  will be flagged as 'underflow', since it parses through the '}' but
  did not open with it.

Max monitoring depth is 60 levels of nesting, to avoid incurring any
memory allocation.