hashes: Move from_engine function to other macro

The `from_engine` function is associated with a general hash type but we
are defining it in the `hash_type` macro which holds nothing but
functions associated with the `Hash` trait. By "associated" I mean
methods on the type as opposed to trait methods.

In preparation for re-naming the `hash_type` macro move the
`from_engine` function there. Requires duplicating the code in the
`siphash` impl block, this is as expected because the `siphash` requires
a custom implementation of the general hashing functionality.

Internal change only.

hashes/src/internal_macros.rs

@@ -130,6 +130,9 @@ macro_rules! hash_type {
         $crate::internal_macros::hash_type_no_default!($bits, $reverse, $doc);
 
         impl Hash {
+            /// Produces a hash from the current state of a given engine.
+            pub fn from_engine(e: HashEngine) -> Hash { from_engine(e) }
+
             /// Constructs a new engine.
             pub fn engine() -> HashEngine { Default::default() }
 
@@ -180,9 +183,6 @@ macro_rules! hash_type_no_default {
                 unsafe { &mut *(bytes as *mut _ as *mut Self) }
             }
 
-            /// Produces a hash from the current state of a given engine.
-            pub fn from_engine(e: HashEngine) -> Hash { from_engine(e) }
-
             /// Copies a byte slice into a hash object.
             pub fn from_slice(
                 sl: &[u8],

hashes/src/siphash24.rs

@@ -168,6 +168,9 @@ impl crate::HashEngine for HashEngine {
 }
 
 impl Hash {
+    /// Produces a hash from the current state of a given engine.
+    pub fn from_engine(e: HashEngine) -> Hash { from_engine(e) }
+
     /// Hashes the given data with an engine with the provided keys.
     pub fn hash_with_keys(k0: u64, k1: u64, data: &[u8]) -> Hash {
         let mut engine = HashEngine::with_keys(k0, k1);