Add System.Memory to the standard packages

[BillWagner]

Found while investigating dotnet/docs#11979 See https://github.com/dotnet/docs/issues/11979#issuecomment-498481429

Now that we've moved to the Blazor implementation, the System.Memory package with Span<T> and related types is not loaded.

These types are useful for many modern scenarios to minimize memory allocations.

See earlier related issues #99 and #85