Add ability to use a statically generated ecmult context.

This vastly shrinks the size of the context required for signing on devices with
memory-mapped Flash.

Tables are generated by the new gen_context tool into a header.

src/group.h

@@ -38,6 +38,8 @@ typedef struct {
 
 #define SECP256K1_GE_STORAGE_CONST(a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p) {SECP256K1_FE_STORAGE_CONST((a),(b),(c),(d),(e),(f),(g),(h)), SECP256K1_FE_STORAGE_CONST((i),(j),(k),(l),(m),(n),(o),(p))}
 
+#define SECP256K1_GE_STORAGE_CONST_GET(t) SECP256K1_FE_STORAGE_CONST_GET(t.x), SECP256K1_FE_STORAGE_CONST_GET(t.y)
+
 /** Set a group element equal to the point at infinity */
 static void secp256k1_ge_set_infinity(secp256k1_ge_t *r);