Rename Digest to Hash

[cespare]

Digest is not a good name. Normally "digest" refers to the output of the hash function.

In v3 we could consider renaming it; perhaps to Hash.

Typical code probably doesn't refer to Digest by name and doesn't have to change:

d := xxhash.New()
... call d.Write() ...
x := d.Sum64()

[greatroar]

Hash also refers the output of a hash function :)

[cespare]

Yes, but it it is also the name of the type of function...as in hash function. (Plus this matches hash.Hash.)

Open to better suggestions though :)