Add Ctypes.string_start.

Ctypes.string_start works like Ctypes.bigarray_start--it returns
a pointer to the start of the string. Existence of this function
is motivated by the C APIs which mutate strings passed to them,
but do not capture the pointers.

In particular, I've worked on wrappers for such libraries as libsodium and libzmq, and they both would benefit from this API.