CSHARP-2758 : Add Span<T> support to ObjectId

[jasonkuo41]

This adds the System.Memory package to the driver.

I hope that we could use this to further boost the performance of the driver with the new concept of Span<T> and Memory<T> introduced in .Net Core 2.1.

I'll add Span support to ObjectId for now, note that it may be possible to also add such support (and many other concepts introduced in System.Memory) towards BsonStream making it more performant in the future.

[jasonkuo41]

As per: https://jira.mongodb.org/browse/CSHARP-2758

[BorisDog]

Thanks for submitting this ticket @jasonkuo41

We are going to introduce support for Span<T> in our serialization in CSHARP-5125. It's scheduled after 3.0 release where we plan to introduce some architectural changes. Closing this ticket as won't as do, as this will be part of CSHARP-5125.