Remove secret-dependant non-constant time operation in ecmult_const.

ECMULT_CONST_TABLE_GET_GE was branching on its secret input.

Also makes secp256k1_gej_double_var implemented as a wrapper
 on secp256k1_gej_double_nonzero instead of the other way
 around.  This wasn't a constant time bug but it was fragile
 and could easily become one in the future if the double_var
 algorithm is changed.

src/util.h

@@ -14,6 +14,7 @@
 #include <stdlib.h>
 #include <stdint.h>
 #include <stdio.h>
+#include <limits.h>
 
 typedef struct {
     void (*fn)(const char *text, void* data);