Document lifetimes & ownership rules

[encukou]

This is already in the devguide.

For new API, the ownership rules and lifetimes of all applicable struct fields, arguments and return values need to be well defined.

Details for the common cases are in #16 (references) and #8 (blueprint structs).

A common case to watch out for is char * and other arrays; especially ones referenced from structs -- AFAIK, in the current API we still sometimes assume they are statically allocated and constant.