Spanning the Void

More minor tweaks for ATS support.

• IP - better handling of sockaddr types.
• MemSpan - better construction for stricter const correctness.
• MemSpan - handle literal strings better by excluding the terminal nul character by default.
• MemSpan - related to above, handle C arrays better.
• MemSpan - add restrict method to optionally shorten the view (change to minimum of current size, restrict value).
• BWF - remove direct support for C strings - depend on MemSpan instead.