Add a C wrapper to compute polygon solid angles

[astrofrog]

As discussed in #3, we can write a routine directly at the C level to compute polygon areas (rather than relying on compute_overlap as in #3).

[cdeil]

Looks like @bernie-simon and @mdboom working on the sphere repo. If this gets included in the Astropy core we could consider using it here (if it has the functionality / speed we want to compute pixel overlaps).