ENHANCED: library(crypto): Faster conversion to bytes when the integer is known.

These entailed constraints only affect performance, and can be removed
without affecting the correctness of the code. They speed up scalar
multiplication of a curve point, used for example in Bitcoinolog.

src/lib/crypto.pl

@@ -713,12 +713,17 @@ curve25519_scalar_mult(Scalar, Point, Result) :-
         '$curve25519_scalar_mult'(ScalarBytes, PointBytes, Result).
 
 bytes_integer(Bs, N) :-
-        foldl(pow, Bs, 0-0, N-_).
+        foldl(pow, Bs, t(0,0,N), t(N,_,_)).
 
-pow(B, N0-I0, N-I) :-
+pow(B, t(N0,P0,I0), t(N,P,I)) :-
+        (   integer(I0) ->
+            B #= I0 mod 256,
+            I #= I0 >> 8
+        ;   true
+        ),
         B in 0..255,
-        N #= N0 + B*256^I0,
-        I #= I0 + 1.
+        N #= N0 + B*256^P0,
+        P #= P0 + 1.
 
 /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    Operations on Elliptic Curves