Create a Cargo feature flag for each sketch

[notfilippo]

Motivation:

1. We can gate compiler warnings only when certain sketches are tying to be compiled on unsupported platforms (e.g. Limitation on Machine Native Endianness #28)
2. We can allow users to pick and choose their sketches without having to compile the whole crate

By default, all sketches should be turned on IMO.

[tisonkun]

default = [ "full" ]

Lol.

Others would be sketch-hll, sketch-tdigest, and internal-xxx for shared dep gates. But we can start with simple hll/cpc/tdigest and only use a namespace prefix when we need it.

[Xuanwo]

Adding full in default can be quiet pain. I prefer to let users to pick sketchs they need.

But we can start with simple hll/cpc/tdigest and only use a namespace prefix when we need it.

This LGTM.

[tisonkun]

Adding full in default can be quiet pain. I prefer to let users to pick sketchs they need.

Yeah .. I like default = [] as well. Explicit over implicit.