Moved descriptor ties to Tie, added TieBinder and TieCompound.

Added¹ Tie, TieBinder, and TieCompound.

Desc:
    moved tie stuff to Tie and TieBinder.

DescCompound:
    merged ref types to DescCompoundRef (only need one kind);
    moved to doc-group descriptor.

aux:
    reinstated; added deque, stack, and unordered_set, removed basic_string.

compound:
    made compound_validate() allow multiple instances again;
    removed compound_tie() and compound_reset() (replaced by
    TieCompound and TieBinder);
    added compound_serial_size() and compound_serial_sequence();
    short-circuit with test for reference equivalence in compound_equal().

test/common/common:
    added Generic::bind().

test/common/nested_comp:
    added bind() member func to data structs, removed tie_d1();
    removed A dupes, relaxed names.

doc:
    added group_assert.dox, group_aux.dox, group_tie.dox,
    and group_serialize.dox.

1: I caved. TieBinder is super handy. More dynamic.