Make parse errors in sqlpp23-ddl2cpp easier to grok

[rbock]

The grammar is pretty big. A small deviation from the supported format typically leads to very long error messages.

Find ways to break this down into smaller, more readable chunks.

[rbock]

Handled in the context of #130.

The primary revelation is that operators + and - both create And expressions, but - suppresses backtracking. Using - essentially means that once the leading expression matches, the trailing has to match. That leads to significantly shorter error messages.