Improve interactive messages and errors

New function: `rxt-syntax-tree-readable' to extract the original
source text from a fragment of the syntax tree when possible.

This is used so far only in the error messages for generating all
productions, when the regexp is unbounded in length.
Rename `rxt-highlight' to `rxt-highlight-face'
Add syntax-based highlight of Elisp regexp at point.
Fix key binding of `rxt-elisp-to-strings' in `rxt-mode-map'
Replace cl-multiple-value-bind with cl-destructuring-bind

cl-lib's `values' is just `list', so we may as well be honest about this
whitespace cleanup
Use a more distinctive face for char-class negation
bump version number, better description
Key binding for pcre query-replace
Improve readme