Geohash encoder and decoder

A geohash names one rectangular cell of the map. Each extra character halves the cell both ways, so precision is a trade: four characters is a city block or two, nine puts a point inside a building. Two hashes that share a prefix sit in neighbouring or nearby cells, but two hashes that share nothing can still be close across a cell edge, which is what the compare mode's overlap and distance figures are for. Cell sizes in metres are spherical approximations, good for choosing a precision, not for surveying.