Move tests after definition of `tf-with-timeout` to fix the use of
a macro before its definition.

(tf--get-secs): New function.
(tf-run-with-idle-timer): Allow `secs` and `redosecs` to be functions
rather than expressions.
(tf--run-while-idle): Rename from `tf-run-while-idle`.
Adjust to new `redosecs` functionality.
(tf--with-timeout-handler-internal): Use `symbol-value` rather than `eval`.
(tf--internal-var-recenter): Rename from tf-internal-var-recenter.

Most of these are minor issues involving, e.g., quoting `like this'
instead of 'like this'.  A few involve escaping ` and ' with a
preceding \= when the characters should not be turned into curved single
quotes.

Fix compiler warnings.