Make new_around_pointer

[mohawk2]

This is inspired by an upcoming paper by @chrisarg - the C API for PDL is quite heavyweight for quite an obviously useful thing.

Another thing that would help generally (including for callbacks) is being able to stack-allocate a struct pdl and then call an inline function (which would live in pdl.h) to initialise it; this could then also be used in pdl_pdlnew itself, so there'd be only one "source of truth" for initialisation. There should also be a slightly more powerful version that also initialises dims and datatype, and a further one that also initialises with data (usable by this).