Use float32 instead of int64 for bucket-keys?

[speezepearson]

Currently, doubles are "packed" into int64s by doing 3 things:

• typecast the double's bytes to an int64
• mask out most of the mantissa
• bitshifting away all of the 0s that we just masked out

This turns the double into a fairly small int64 (IIUC, we care about "small" because it takes advantage of Protobuf's varint encoding).

Here's another approach, which might be preferable:

• convert the double to a float
• mask out most of the mantissa

The main differences between the two approaches are:

• Range of values expressible. float has only 8 exponent bits, not 11, so it can only represent values in the range 2^(±128) instead of 2^(±1024). This seems inconsequential.
• Size of representation. floats take 4 bytes; varints take less, usually, but in practice, all reasonably-sized doubles (other than exactly 0) pack to ints between 2^14 and 2^21, and therefore take 3 bytes (demo). So using floats instead would only make most bucket keys 33% larger.
• Simplicity. Storing data in its natural datatype is good. The type makes it clear what's being stored, and the strings produced by languages' built-in stringification methods aren't opaque.

[speezepearson]

(spawned by discussion with @orborde)