[crypto] Add bigint_copy() as a convenient wrapper macro

Big integers may be efficiently copied using bigint_shrink() (which
will always copy only the size of the destination integer), but this
is potentially confusing to a reader of the code.

Provide bigint_copy() as an alias for bigint_shrink() so that the
intention of the calling code may be more obvious.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/bigint.h

@@ -8,6 +8,8 @@
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
+#include <assert.h>
+
 /**
  * Define a big-integer type
  *
@@ -176,6 +178,17 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 			    (dest)->element, dest_size );		\
 	} while ( 0 )
 
+/**
+ * Copy big integer
+ *
+ * @v source		Source big integer
+ * @v dest		Destination big integer
+ */
+#define bigint_copy( source, dest ) do {				\
+	build_assert ( sizeof ( *(source) ) == sizeof ( *(dest) ) );	\
+	bigint_shrink ( (source), (dest) );				\
+	} while ( 0 )
+
 /**
  * Multiply big integers
  *