Efficient cdef calling convention for buffers

[dagss]

Currently there's no way of creating efficient functions taking a buffer argument -- for starters cdef functions doesn't support declaring an argument as a buffer, and even if it did, it would mean a costly (in this context) buffer acquisition on every call.

An attractive alternative would be to have

cdef utilfunc(object[int] buf): ...

mean a different calling convention, where the caller must acquire the buffer, and a struct with the necesarry information is passed. If the caller already has a buffer then it can simply pass the information it already has.

(There's a subtlety here: If buf is reassigned to in utilfunc, then the original buffer must not be released, while the new buffer must be released eventually.)

[dagss]

One can simply disable reassigning to the argument in the function initially.

[scoder]

Also see #180, #340.

[robertwb]

This is also related to being able to handle buffers from closures #660.