datum type comparison ops

[RhodiumToad]

Users are surprised that == does not work for datum types (other than Numeric and types auto-converted to Lua values). Consider adding :equal method and __eq metamethod to all datum types that have a default btree or hash opclass. Also __lt and friends for types with a btree opclass?

[bjne]

This sounds great, and I vote for the shortforms; :eq :lt :gt