Access all hashes and MACs through the standard API.

All the hash-specific state structures, and the functions that
directly accessed them, are now local to the source files implementing
the hashes themselves. Everywhere we previously used those types or
functions, we're now using the standard ssh_hash or ssh2_mac API.

The 'simple' functions (hmacmd5_simple, SHA_Simple etc) are now a pair
of wrappers in sshauxcrypt.c, each of which takes an algorithm
structure and can do the same conceptual thing regardless of what it
is.

sshmd5.c

@@ -6,6 +6,18 @@
  * Simon Tatham.
  */
 
+typedef struct {
+    uint32_t h[4];
+} MD5_Core_State;
+
+struct MD5Context {
+    MD5_Core_State core;
+    unsigned char block[64];
+    int blkused;
+    uint64_t len;
+    BinarySink_IMPLEMENTATION;
+};
+
 /* ----------------------------------------------------------------------
  * Core MD5 algorithm: processes 16-word blocks into a message digest.
  */