Implement pcre-mode using advice instead of remapping
Basic isearch support in pcre-mode
Use an error symbol instead of (error ...)

The new error symbol, 'rxt-invalid-regexp has 'invalid-regexp as its
"superclass", allowing isearch-mode to recognise pcre-translation errors
as invalid regexps.
Comment/whitespace cleanup
Fix missing argument typo
Hack to isearch-fallback
Fontification hackery in isearch-message-prefix advice
Improve Isearch in pcre-mode

- Use isearch-mode-hook and isearch-mode-end-hook to add and remove the
  custom search callback

- Cache converted regexps to avoid re-computing on each search

- Signal an error on regexps with a trailing backslash, which prevents
  isearch from prematurely moving ahead to a literal backslash in the
  buffer.  Probably this case should be caught in the main
  rxt-pcre-to-elisp function.
Re-implement pcre-query-replace-regexp using pcre-mode

If the new pre-mode works well this function will probably go away at
some point.
Tweaks to pcre-mode comments & prompts
Add cache and improve query-replace-regexp messages.
Clean up comments & docstrings
Merge branch 'isearch-advice'