Make integer handles use non-zero types

[madsmtm]

In similar spirit as #136.

I'm not sure myself that this is such a good idea, mainly because:

• The c_int/c_long types are not possible to convert to non-zero types in the same manner (leading to us only using the API in some places).
• Ensuring non-null pointers is important, since that causes unsoundness if used incorrectly, while a zero handle isn't that big of a problem, any operations you do on them will just error (just like any other invalid handle).

Other options would be to panic on 0 in the constructor (a weak check against invalid handles, assuming those are exceedingly rare), or we could document that it is a bug in the implementer (as opposed to the consumer) to provide zero handles where unexpected?

[notgull]

Looks good to me!

[madsmtm]

Uh, I kinda wanted to debate this after #136, but oh well

[notgull]

Oh, my bad, there weren't any comments for a while so I assumed that the debate was settled.