Use Span<T> for APIs

[Marioalexsan]

Supersedes #263.

To recap, what we want to do is:

• Multitarget .NET Standard 2.0 and .NET Standard 2.1, and install System.Memory on NS2.0 to gain access to Span there
• Replace T[] with Span in input parameters where possible - this is a trivial change that just allows slicing
• Replace T[] returns with Span where possible (i.e. when we want to just give a view into memory) - this reduces the number of allocations and solves some anti-patterns like Image.Pixels
• Prefer ReadOnlySpan over Span where possible

In some cases, the implementation has to be split in NS2.0 and NS2.1 specific sections to be able to use Span-enhanced methods from .NET. One such example is in StreamAdaptor's Read method, where you can read directly into a Span on NS2.1.

Things to consider:

• Spans must be used carefully with memory that can get invalidated from the C/C++ side, so we have to make sure that such memory will never be freed / moved for places where we just take the pointers and convert them to spans