add desvar_meta and constraint_meta to recorder metadata

[fzahle]

the current Unknowns metadata passed to recorders only contain the entry is_desvar or is_constraint, but doesn't allow the user to check whether the constraint is violated or not, since bounds are not recorded anywhere.

adding desvar_meta and constraint_meta with the dicts for each desvar/constraint now makes this possible.

The is_desvar or is_constraint are sort of obsolete but kept for backward compatibility.