Abit - functions to work with :atomics as a bit array (?)

I have added a module Abit.Counter.
This is similar to Erlang :counters but it allows you to use counters with smaller than 64 bit sizes:

bits | unsigned value range | signed value range
2      0..3                   -2..1
4      0..15                  -8..7
8      0..255                 -128..127
16     0..65535               -32768..32767
32     0..4294967295          -2147483648..2147483647

So for example if you only need max value 255 (8 bits) unsigned, you can have atomics_size * 8 counters instead of just atomics_size.

Here is the short summary of the API of Abit.Counter module:

Abit.Counter

• Abit.Counter.new/2 - Create a new array of counters. Returns %Abit.Counter{} struct.
• Abit.Counter.get/2 - Returns the value of counter at the given index.
• Abit.Counter.put/3 - Puts the value into counter at the given index.
• Abit.Counter.add/3 - Adds the increment to counter at the given index.