digest: move EagerHash from hmac

Originally added in RustCrypto/MACs#151

The situation is `ecdsa` has a trait for the "default" hash function to
use for a particular curve, which we want to work with RFC6979 and ergo
HMAC, so we would like to use an `EagerHash` bound.

But `rfc6979` is an optional dependency, so we can't depend on its
transitive accessibility to `rfc6979::hmac`. This PR added `hmac` as a
dependency gated on the `digest` feature of `ecdsa`:

RustCrypto/signatures#1076

But as I was looking at `EagerHash` I noticed there's nothing
HMAC-specific about it at all and it can easily be moved to `digest`, so
we're able to use it in bounds without any other dependencies besides
`digest`, which would be nice.

Author: tarcieri

digest/src/block_api.rs

@@ -3,7 +3,7 @@
 //! Usage of traits in this module in user code is discouraged. Instead use
 //! core algorithm wrapped by the wrapper types, which implement the
 //! higher-level traits.
-use crate::InvalidOutputSize;
+use crate::{Digest, HashMarker, InvalidOutputSize};
 
 pub use block_buffer::{Eager, Lazy};
 pub use crypto_common::{AlgorithmName, Block, BlockSizeUser, OutputSizeUser, Reset};
@@ -30,6 +30,32 @@ pub trait BufferKindUser: BlockSizeUser {
     type BufferKind: BufferKind;
 }
 
+/// Trait implemented by eager hashes which expose their block-level core.
+pub trait EagerHash: BlockSizeUser + Digest {
+    /// Block-level core type of the hash.
+    type Core: HashMarker
+        + UpdateCore
+        + FixedOutputCore
+        + BlockSizeUser<BlockSize = <Self as BlockSizeUser>::BlockSize>
+        + BufferKindUser<BufferKind = Eager>
+        + Default
+        + Clone;
+}
+
+impl<T> EagerHash for T
+where
+    T: CoreProxy + BlockSizeUser + Digest,
+    <T as CoreProxy>::Core: HashMarker
+        + UpdateCore
+        + FixedOutputCore
+        + BlockSizeUser<BlockSize = <Self as BlockSizeUser>::BlockSize>
+        + BufferKindUser<BufferKind = Eager>
+        + Default
+        + Clone,
+{
+    type Core = T::Core;
+}
+
 /// Core trait for hash functions with fixed output size.
 pub trait FixedOutputCore: UpdateCore + BufferKindUser + OutputSizeUser {
     /// Finalize state using remaining data stored in the provided block buffer,