Small performance cleanups in S.S.Cryptography

src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/ManagedX509ExtensionProcessor.cs

@@ -31,7 +31,7 @@ public virtual void DecodeX509KeyUsageExtension(byte[] encoded, out X509KeyUsage
 
             try
             {
-                AsnReader reader = new AsnReader(encoded, AsnEncodingRules.BER);
+                AsnValueReader reader = new AsnValueReader(encoded, AsnEncodingRules.BER);
                 keyUsagesAsn = reader.ReadNamedBitListValue<KeyUsageFlagsAsn>();
                 reader.ThrowIfNotEmpty();
             }

src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/OpenSslPkcs12Reader.cs

@@ -57,6 +57,7 @@ protected override AsymmetricAlgorithm LoadKey(ReadOnlyMemory<byte> pkcs8)
 
             if (bytesRead != pkcs8.Length)
             {
+                key.Dispose();
                 throw new CryptographicException(SR.Cryptography_Der_Invalid_Encoding);
             }
 

src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X509Certificates/RSAPkcs1X509SignatureGenerator.cs

@@ -30,6 +30,7 @@ protected override PublicKey BuildPublicKey()
         internal static PublicKey BuildPublicKey(RSA rsa)
         {
             Oid oid = Oids.RsaOid;
+            ReadOnlySpan<byte> asnNull = new byte[] { 0x05, 0x00 };
 
             // The OID is being passed to everything here because that's what
             // X509Certificate2.PublicKey does.
@@ -39,7 +40,7 @@ internal static PublicKey BuildPublicKey(RSA rsa)
                 //
                 // This is due to one version of the ASN.1 not including OPTIONAL, and that was
                 // the version that got predominately implemented for RSA. Now it's convention.
-                new AsnEncodedData(oid, stackalloc byte[] { 0x05, 0x00 }),
+                new AsnEncodedData(oid, asnNull),
                 new AsnEncodedData(oid, rsa.ExportRSAPublicKey()));
         }