crypto, zlib: change _binding to _handle for consistency

[seishun]

As mentioned in #7372, replacing all instances of _binding with _handle for consistency with the rest of node's internal code.

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[indutny]

cc @tjfontaine , that's on your field

[trevnorris]

Going to say this is OK. Using _handle is the convention for any objects that have a C++ class counterpart that can be extracted using Unwrap<T>().

@tjfontaine Anything I'm missing?

[tjfontaine]

Yup, LGTM

[trevnorris]

Thanks. Merged in 5344d0c.