Restrict generic key type to only those supported

[pete-woods]

Without this, you can specify key types that aren't supported and then have to deal with panics, rather than more pleasant compile-time warnings.

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[mangalaman93]

Thanks for the PR. The only problem that I see with this is that any new type addition requires changes to the code and upgrading ristretto.

[pete-woods]

Thanks for the PR. The only problem that I see with this is that any new type addition requires changes to the code and upgrading ristretto.

I believe this is already the case (see linked code): https://github.com/dgraph-io/ristretto/blob/main/z/z.go#L55

Also I put the type next to the key mapping func, so it's clear they both need updating in lockstep.

[pete-woods]

Thanks for your time! 🙇