chibi.crypto: make Chibi use native SHA-2 by default

This change concerns only Chibi. The portable implementation is
still kept around because it is... well... portable and can be
used by other Scheme implementations.

lib/chibi/crypto/sha2.sld

@@ -1,8 +1,14 @@
 
 (define-library (chibi crypto sha2)
-  (import (scheme base) (srfi 33) (chibi bytevector))
   (export sha-224 sha-256)
-  (include "sha2.scm"))
+  (cond-expand
+   (chibi
+    (import (scheme base))
+    (include "sha2-native.scm")
+    (include-shared "crypto"))
+   (else
+    (import (scheme base) (srfi 33) (chibi bytevector))
+    (include "sha2.scm"))))
 
 ;;> \procedure{(sha-224 src)}
 ;;>