Add Customized trait for blake2

[sylvainpelissier]

Allow to use Blake2 with a customized string: Blake2b128::new_customized(persona) as described in the paper and as for cSHAKE. It needs a new trait VarOutputCustomized implemented by: RustCrypto/traits#1787

[newpavlov]

You should add a patch for digest using the traits repo. Add this to the root Cargo.toml:

[patch.crates-io]
# https://github.com/RustCrypto/traits/pull/1787
digest = { git = "https://github.com/RustCrypto/traits" }