Reorganize delay_partial_inst.m before working on it.

compiler/delay_partial_inst.m:
    Separate out the important code: handling constructions and deconstructions
    involving partially instantiated terms.

    Put the predicates in a logical order.

compiler/hlds_goal.m:
    Provide insts for each kind of unification.

    Remove some existing insts that were redundant.

compiler/accumulator.m:
compiler/distance_granularity.m:
compiler/higher_order.m:
compiler/lco.m:
compiler/structure_reuse.indirect.m:
    Conform to the change to hlds_goal.m.