refactor API

[maedoc]

• [x] move sys, hist, rng to scheme
• [x] use conn everywhere instead of nnz, Or, Ic, w, d
• [x] drop const, restrict and typedef
• [x] move aff eff state vectors to scheme
• [x] consistent dim names, n_dim, n_in, n_out without s
• [x] API clients handle I/O not us (event. integrate some r/w interface but not now)
• [x] s/void * ptr/void *data/ & make it always first member of struct
• [x] make interface "cast"s explicit e.g. as_sys instead of get_sys to mark that it's the same object
• [x] consistent object model (free, n_byte, copy)
• [x] consistent mem policy (always copy constructor arg e.g.) and document
• [x] update impl & tests to follow API
• [ ] decide (& impl) copy is shallow or deep?

"A [reasonable] consistency is the [start of a great API]" said Emerson.