Move crypto into its own subdirectory.

Similarly to 'utils', I've moved all the stuff in the crypto
build-time library into a source directory of its own, and while I'm
at it, split up the monolithic sshauxcrypt.c into its various
unrelated parts.

This is also an opportunity to remove the annoying 'ssh' prefix from
the front of the file names, and give several of them less cryptic
names.

crypto/hash_simple.c

@@ -0,0 +1,13 @@
+/*
+ * Convenience function to hash a single piece of data, wrapping up
+ * the faff of making and freeing an ssh_hash.
+ */
+
+#include "ssh.h"
+
+void hash_simple(const ssh_hashalg *alg, ptrlen data, void *output)
+{
+    ssh_hash *hash = ssh_hash_new(alg);
+    put_datapl(hash, data);
+    ssh_hash_final(hash, output);
+}