Functional search_rtree on external memory

[H-Plus-Time]

There's a fair amount of change overlap with #55 , the relevant changes are the exported search function in python/src/rtree.rs.

It should accept every buffer protocol object marked as a 1D uint8 array (I've tested with bytes, bytearray, array, memoryview, geoindex_rs.RTree, 1D numpy u1 arrays, all working identically), and raise more or less immediately (importantly, before the unsafe transmute) on anything else.

Parity for KDTree doesn't look terribly difficult, and by the looks of it the api surface is straightforward (prepend everything with the target buffer, basically).

[kylebarron]

@H-Plus-Time let me know what you think of #57 . I think it's a bit cleaner