Handle entries of multiple symbols in electric-layout-rules
Instead of allowing multiple rules in electric-layout-rules, allow
rules to specify multiple symbols. This should be entirely
backward-compatible to existing customizations of that variable.
* lisp/electric.el (electric-layout-rules): a single entry can
specify multiple symbols.
(electric-layout-post-self-insert-function-1): rework.
* test/lisp/electric-tests.el: Update tests to work with new semantics
of electric-layout-rules.