generator: speed up parsing

Similar to speeding up serialization; in our parsing logic we did a
bunch of expensive stuff then expensively inverted it. Drop everything
except the essential checks and then memcpy.

Author: apoelstra

src/modules/generator/main_impl.h

@@ -288,10 +288,8 @@ int secp256k1_pedersen_commitment_parse(const secp256k1_context* ctx, secp256k1_
         !secp256k1_ge_set_xquad(&ge, &x)) {
         return 0;
     }
-    if (input[0] & 1) {
-        secp256k1_ge_neg(&ge, &ge);
-    }
-    secp256k1_pedersen_commitment_save(commit, &ge);
+
+    memcpy(commit->data, input, 33);
     return 1;
 }