Add basic Span<T> support

[DaZombieKiller]

This pull request adds basic support for Span<T> and ReadOnlySpan<T> to the AsmResolver and AsmResolver.IO namespaces.

Support is only implemented for .NET Standard 2.1 and higher, no attempt has been made to provide Span<T> support for .NET Standard 2.0 as it would introduce a dependency on System.Memory.

This PR also does not attempt to take advantage of this new support within the library. This is most likely better left to a separate PR (and/or set of issues).

The following new types are provided to avoid source and binary breaking changes in existing code:

namespace AsmResolver.IO
{
    public interface ISpanDataSource : IDataSource
    {
        int ReadBytes(ulong address, Span<byte> buffer);
    }

    public interface ISpanBinaryStreamWriter : IBinaryStreamWriter
    {
        void WriteBytes(ReadOnlySpan<byte> buffer);
    }

    public static class DataSourceExtensions
    {
        public static int ReadBytes(this IDataSource dataSource, ulong address, Span<byte> buffer);
    }
}

ISpanDataSource and ISpanBinaryStreamWriter can be considered temporary, as they only exist to avoid the breaking change of adding a new member to the corresponding IDataSource and IBinaryStreamWriter interfaces. In the future, it may be desirable to remove them and take the break.

[ds5678]

It might be a good idea to add a define variable, eg SPAN_SUPPORTED, instead of using NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_1_OR_GREATER for the preprocessor directives.

[DaZombieKiller]

@ds5678

It might be a good idea to add a define variable, eg SPAN_SUPPORTED, instead of using NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_1_OR_GREATER for the preprocessor directives.

I thought about doing this, but decided to wait for feedback to see what would be preferred. Since there aren't any plans to expose this under .NET Standard 2.0, SPAN_SUPPORTED will never mean anything other than NETSTANDARD2_1_OR_GREATER || NETCOREAPP2_1_OR_GREATER, but I suppose it is much less noisy in the source.

[Washi1337]

I have no real strong opinions on introducing either SPAN_SUPPORTED or leaving it as an OR expression as it is now. If it is easy to add SPAN_SUPPORTED without too much messing around in the csprojs, then I'm all for it. If not, we can leave it as is as well.

[Washi1337]

I just realized your PR is based on master. Can you please change the base branch to development and see if all the tests still succeed?

[Washi1337]

Thanks!