Generate ecmult_static_pre_g.h

This header contains a static array that replaces the ecmult_context pre_g and pre_g_128 tables.
The gen_ecmult_static_pre_g program generates this header file.

src/ecmult.h

@@ -11,6 +11,24 @@
 #include "scalar.h"
 #include "scratch.h"
 
+/* Noone will ever need more than a window size of 24. The code might
+ * be correct for larger values of ECMULT_WINDOW_SIZE but this is not
+ * not tested.
+ *
+ * The following limitations are known, and there are probably more:
+ * If WINDOW_G > 27 and size_t has 32 bits, then the code is incorrect
+ * because the size of the memory object that we allocate (in bytes)
+ * will not fit in a size_t.
+ * If WINDOW_G > 31 and int has 32 bits, then the code is incorrect
+ * because certain expressions will overflow.
+ */
+#if ECMULT_WINDOW_SIZE < 2 || ECMULT_WINDOW_SIZE > 24
+#  error Set ECMULT_WINDOW_SIZE to an integer in range [2..24].
+#endif
+
+/** The number of entries a table with precomputed multiples needs to have. */
+#define ECMULT_TABLE_SIZE(w) (1L << ((w)-2))
+
 typedef struct {
     /* For accelerating the computation of a*P + b*G: */
     secp256k1_ge_storage (*pre_g)[];    /* odd multiples of the generator */