fix: more idiomatic elisp, better docstrings, less byte-compiler warnings

By Stefan Monnier, with original commentary.

"Here's an untested patch of cleanups, found partly via compilation using
lexical-binding and partly via checkdoc-current-buffer (and
occasionally by stumble-upon happenstance)."
fix: replace 'text' in 'yas-text' in bundled snippets

The previous commit broke these snippets by enabling
Fix: cleanup yasnippet-tests.el

- add edebug-form-specs
- replace "yas/" with "yas-"
- yas/ert => yas-batch-run-tests
Closes #318: correctly implement YAS--MODES-TO-ACTIVATE

- simpler and got rid of YAS--ALL-PARENTS
- also got rid of YAS/MODE-SYMBOL backward compatibility hack
Closes #318: Don't use CL-LABELS for backward compatibility with older emacsen
fix: shoosh warning with LETREC instead of LET
fix: please older emacsen, don't use LETREC after all

also fix some compiler warnings