Puf strengthen types

[puf]

Description

This PR makes the GeoFire API more type-safe by introducing three new types:

• type geopoint = [number, number], replacing number[] everywhere it represents a single geopoint.
• type geohash = string, replacing string everywhere it represents a geohash value.
• type geohashRange = [geohash, geohash] replacing string[] where it represents a tuple of two strings representing a geohash range to query.

This change does not make any change to the actual underlying code, and the signatures are backwards compatible to any correctly working code on the previous API. The only difference after this change is that more errors will be caught at compile time (when TypeScript is converted to JavaScript), rather than by the extensive runtime checks that GeoFire performs on them.

For googlers: the full list of type changes can be found on go/geofire-types.

[coveralls]

Coverage increased (+0.3%) to 92.202% when pulling 49c1d04be9f714271339c33b92ea0539bd605544 on puf-strengthen-types into 8d13bcd3df606cebfb20168e448033d3cc98f8ae on master.

[puf]

Don't merge yet, as I think I still need to move some tests - which should also improve coverage again.

But aside from that, the main code could do with a thorough review.

[puf]

OK, I think that covers it for now. @samtstern : Can you review, mostly focusing on the signature changes of the API to ensure they're indeed non-breaking? See go/geofire-types 🔐 for reference of the changes.