Purged ZStringDesc; made Desc only tie to char strings.

DescType:
    removed zstring.

DescData:
    removed ZStringDesc.

Desc:
    made std::basic_string ties only accept <char, ...>¹.

compound:
    only take ref to DescCompoundRef instead of rvalue ref².

1: May later permit UTF-16 and UTF-32 strings, but for now
assuming pre-encoded, valid UTF-8 on both ends.

2: Avoid invoking move ctor, even though this will always
be an rvalue ref.
Removed aux.hpp, String.hpp, and doc/pages/group_aux.dox.
Made desc data class size properties uint32_t; added Desc::get_tie().

DescType: explicitly begin enum at 0u.

DescData:
    made size properties of StringDesc and ArrayDesc uint32_t
    instead of arch-dependent std::size_t.

Desc: added get_tie().