generator: massively speed up serialization

`secp256k1_pedersen_commit_serialize` would call `_load` (which does a
sqrt to fully decompress the key, then a conditional negation based on
the flag), then check the Jacobian symbol of the resulting y-coordinate,
then re-serialize based on this.

Instead, don't do any of this stuff. Copy the flag directly out of the
internal representation and copy the x-coordinate directly out of the
internal representation.

Checked that none of the other _serialize methods in the modules do
this.

Fixes BlockstreamResearch#293

src/modules/generator/main_impl.h

@@ -296,17 +296,11 @@ int secp256k1_pedersen_commitment_parse(const secp256k1_context* ctx, secp256k1_
 }
 
 int secp256k1_pedersen_commitment_serialize(const secp256k1_context* ctx, unsigned char *output, const secp256k1_pedersen_commitment* commit) {
-    secp256k1_ge ge;
-
     VERIFY_CHECK(ctx != NULL);
     ARG_CHECK(output != NULL);
     ARG_CHECK(commit != NULL);
 
-    secp256k1_pedersen_commitment_load(&ge, commit);
-
-    output[0] = 9 ^ secp256k1_fe_is_square_var(&ge.y);
-    secp256k1_fe_normalize_var(&ge.x);
-    secp256k1_fe_get_b32(&output[1], &ge.x);
+    memcpy(output, commit->data, 33);
     return 1;
 }
 

src/modules/generator/tests_impl.h

@@ -264,7 +264,13 @@ static void test_pedersen(void) {
     }
     CHECK(secp256k1_pedersen_blind_sum(CTX, &blinds[(total - 1) * 32], bptr, total - 1, inputs));
     for (i = 0; i < total; i++) {
+        unsigned char result[33];
+        secp256k1_pedersen_commitment parse;
+
         CHECK(secp256k1_pedersen_commit(CTX, &commits[i], &blinds[i * 32], values[i], secp256k1_generator_h));
+        CHECK(secp256k1_pedersen_commitment_serialize(CTX, result, &commits[i]));
+        CHECK(secp256k1_pedersen_commitment_parse(CTX, &parse, result));
+        CHECK(secp256k1_memcmp_var(&commits[i], result, 33) == 0);
     }
     CHECK(secp256k1_pedersen_verify_tally(CTX, cptr, inputs, &cptr[inputs], outputs));
     CHECK(secp256k1_pedersen_verify_tally(CTX, &cptr[inputs], outputs, cptr, inputs));