adds support for generic counters

[sunny-g]

No description provided.

[chpio]

What is the purpose/usecase for this? Saving on storage capacity if you only need smaller counters? Other types than ints like enums etc don't make much sens (besides maybe newtypes/int-wrappers).

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is MVReg::<_, _, u64> necessary in the code? C defaults to 64, so it shouldn't.

[sunny-g]

@chpio

1. purpose is so that I can use the BigUint as a counter value in PNCounters.
2. it shouldn't be required b/c of the default as you stated, but for whatever reason it wouldn't compile. This isn't quite ready for merging yet as I want to fix that issue, and ensure that I'm not imposing stricter trait constraints than necessary on that generic throughout the library.