Reduce allocations during ECDSA signing

Reduce the number of byte arrays allocated by using big.Int.FillBytes
when calculating ECDSA signature.

After this change, Benchmarks of ES256 signing method consistently
report 4 fewer allocations.

Before:
```
BenchmarkECDSASigning/Basic_ES256-8              190572         6702 ns/op       4249 B/op         65 allocs/op
BenchmarkECDSASigning/Basic_ES256/sign-only-8     47383        24650 ns/op       3329 B/op         43 allocs/op
```

After:
```
BenchmarkECDSASigning/Basic_ES256-8              187682         6725 ns/op       4121 B/op         61 allocs/op
BenchmarkECDSASigning/Basic_ES256/sign-only-8     48656        24446 ns/op       3201 B/op         39 allocs/op
```

Author: jkanywhere

ecdsa.go

@@ -128,18 +128,12 @@ func (m *SigningMethodECDSA) Sign(signingString string, key interface{}) (string
 			keyBytes += 1
 		}
 
-		// We serialize the outpus (r and s) into big-endian byte arrays and pad
-		// them with zeros on the left to make sure the sizes work out. Both arrays
-		// must be keyBytes long, and the output must be 2*keyBytes long.
-		rBytes := r.Bytes()
-		rBytesPadded := make([]byte, keyBytes)
-		copy(rBytesPadded[keyBytes-len(rBytes):], rBytes)
-
-		sBytes := s.Bytes()
-		sBytesPadded := make([]byte, keyBytes)
-		copy(sBytesPadded[keyBytes-len(sBytes):], sBytes)
-
-		out := append(rBytesPadded, sBytesPadded...)
+		// We serialize the outputs (r and s) into big-endian byte arrays
+		// padded with zeros on the left to make sure the sizes work out.
+		// Output must be 2*keyBytes long.
+		out := make([]byte, 2*keyBytes)
+		r.FillBytes(out[0:keyBytes]) // r is assigned to the first half of output.
+		s.FillBytes(out[keyBytes:])  // s is assigned to the second half of output.
 
 		return EncodeSegment(out), nil
 	} else {