Trying to allow Int8 as indices

[simone-silvestri]

This PR is trying to solve #370. This is just an initial proposal, but I guess there is a lot of work to do still.
I have noticed that

julia> BigInt(1) !== BigInt(1)
true

so I had to manually change some tests when the offsets are BigInts

Any suggestion or comment is more than welcome as I am not very sure about downstream implications of this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.62%. Comparing base (f9fd311) to head (b645e13).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
+ Coverage   96.57%   96.62%   +0.05%     
==========================================
  Files           6        6              
  Lines         467      474       +7     
==========================================
+ Hits          451      458       +7     
  Misses         16       16              

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[giordano]

I'll defer the decision about adding a new type parameter to the maintainers of the package