Now that we have the character sets as parameters (and thus are available to
any other type that may need it), we no longer have subtypes for TEXT.
I propose that the default subtype for TEXT be PLAIN. All other subtypes will
be the subtypes of TEXT-PLUS, and the TEXT-PLUS type abolished. Syntactical
sugar, yes, but it's one less type to worry about and it consolidates things
in a nicer way. There are too many cases where I have to check for TEXT or
TEXT-PLUS which would be simplified by a single TEXT check.