Permit users to specify which fields are significant for hash and equality

[gafter]

We have a number of comments in our code of the form

# TODO(NHD): @auto_hash_equals (currently can't b/c of existing hash that <something>)

If the user could specify which fields are significant, the hash and equality could be automatic. Perhaps something like this:

@auto_hash_equals (:name, :node_id) mutable struct Var <: Term
    name :: Id
    annos :: Annos
    node_id :: NodeId
end

/cc @NHDaly

[NHDaly]

cool! :) Thanks neal, sounds like a nice improvement.

[gafter]

This is now done.

@auto_hash_equals fields=(x, y) struct Point ... end

or, if you want the hash value cached,

@auto_hash_equals cache=true fields=(x, y) struct Point ... end