test/desc/desc: corrected zstring type in tie-constructed compound.
Shuffled code, added equality operations; see full commit message.

Moved DescCompound to DescCompound.hpp.
Moved DescData and data descriptors to DescData.{hpp,cpp}.
Moved compound operations to compound.hpp.

Desc:
    added equality operators;
    renamed get_data() to get_data_union();
    resurrected assertive get_data<DescT>().

{ZString,String,Array,Ref}Desc:
    added equality operators.

utility:
    added bounded_array_cref;
    added equality operators to bounded_array class templates.

compound:
    added compound_equal().

test/desc/desc:
    added compound_equal() test.

doc:
    tidy;
    added missing name groups.
doc: tidy, added missing docs; added group page stubs; hid `detail/`.