add operator for group of related definitions

[rme]

It would be nice to extend the definition operator, or add a new definitions operator, that could be used when documenting a group of related things.

For example, in CCL, the functions %get-byte, %get-unsigned-byte, %get-word, etc. could benefit from being listed inside a single definitions clause, rather than separate ones.

For an example of what I mean, see page 463 in CLTL2, where Steele documents string-lessp, string-greaterp, etc.