Deprecate use of multiple character argument for the attribute (class) opcode

[bertfrees]

If you want to set a character attribute on a list of characters you can currently do it in two ways:

class vowel aeiuo

or

class vowel a
class vowel e
class vowel i
class vowel u
class vowel o

Being able to do it in only one way is better IMO. It is simpler to understand and document and takes away any confusion. We have other opcodes that accept multiple characters but where it has the meaning of a sequence of those characters.