WIP: Faster countmap for small bit type e.g. Int8, UInt8, UInt16, Int16, Float16

[xiaodaigh]

These bits can only have 256 or 65536 distinct values. Instead of using (radix)sorting or Dict one can simply use an array (of size 256 or 65536) to hold and accumulate the results.

From my own testing it will result in 9x-3x speed up. See this gist for example implementation

[nalimilan]

Makes sense. Feel free to make a PR. Having a special case for Float16 doesn't sound worth it, though, it doesn't seem very common to compute the frequency of floating point numbers, and Float16 is a very specific type.

[xiaodaigh]

Implemented in #339

[xiaodaigh]

merged #339