Centralised HMAC implementation.

This replaces all the separate HMAC-implementing wrappers in the
various source files implementing the underlying hashes.

The new HMAC code also correctly handles the case of a key longer than
the underlying hash's block length, by replacing it with its own hash.
This means I can reinstate the test vectors in RFC 6234 which exercise
that case, which I didn't add to cryptsuite before because they'd have
failed.

It also allows me to remove the ad-hoc code at the call site in
cproxy.c which turns out to have been doing the same thing - I think
that must have been the only call site where the question came up
(since MAC keys invented by the main SSH-2 BPP are always shorter than
that).

marshal.h

@@ -31,10 +31,16 @@ struct BinarySink {
  * put 'BinarySink_DELEGATE_IMPLEMENTATION' in its declaration, and
  * when an instance is set up, use 'BinarySink_DELEGATE_INIT' to point
  * at the object it wants to delegate to.
+ *
+ * In such a delegated structure, you might sometimes want to have the
+ * delegation stop being valid (e.g. it might be delegating to an
+ * object that only sometimes exists). You can null out the delegate
+ * pointer using BinarySink_DELEGATE_CLEAR.
  */
 #define BinarySink_DELEGATE_IMPLEMENTATION BinarySink *binarysink_
 #define BinarySink_DELEGATE_INIT(obj, othersink) \
     ((obj)->binarysink_ = BinarySink_UPCAST(othersink))
+#define BinarySink_DELEGATE_CLEAR(obj) ((obj)->binarysink_ = NULL)
 
 /*
  * The implementing type's write function will want to downcast its