Generic enhancements to elisp style and regexp readability

I've replaced some of the long regexps with rx macro in order to enhance readability. Most of the new replacements upon expansion produce the same regexps, up to conversion of escape sequences and reordering of character set elements, but there're some regexps that were simplified a bit. I've checked that expansions correspond to originals but it would be great if someone could test complete functionality in order to see if something is broken.

Also if expressions with only one alternative were converted into when expressions in order to be more lispy.