Improve macOS certificate store

[randombit]

Two major ones are

• Refactor and optimize Windows certificate store #5539 adds a certificate store cache to the Windows certificate store so that when a root is used repeatedly the caller gets a shared copy f the certificate rather than a newly reparsed one. This avoids both parsing overhead and reduces memory consumption.
• Now Certificate_Store directly offers contains as a function which can be overridden. The default implementation is find-all + a linear comparison, which is not very efficient.

There are probably other minor things too (eg missing kSecAttrSerialNumber encoding, if solved, would allow perhaps faster searching)