feat(core): Makes spanwriter resizable (#376)

SpanWriter now has an argument that allows it to be resizable.
```cs
public SpanWriter(Span<byte> initialBuffer, bool resize = false)
public SpanWriter(int initialCapacity, bool resize = false)
```

If a SpanWriter is set to be resizable, or given an initial capacity instead of a buffer, it must be disposed:
```cs
public static void SomeMethod()
{
    using var writer = new SpanWriter(stackalloc byte[512], true);
    // stuff
    
    // Automatic Dispose of writer
}
```

This is because the SpanWriter uses `ArrayPool<byte>.Shared` _rented buffers_ to resize. If the writer is not disposed, those buffers will never be reused resulting in a _memory leak_.

It is possible that the SpanWriter will outright ditch the initial buffer if resize is set to true and growing is needed. To check/account for that we can do the following:

```cs
public static void SomeMethod()
{
    Span<byte> span = stackalloc byte[512];
    using var writer = new SpanWriter(span, true);
    // write some stuff that causes the span to grow
    
    span = writer.RawBuffer;
    // Do stuff with the span
}
```

Make sure you don't accidentally `Dispose()` the writer or use the `RawBuffer` outside of the `using` block. If you do, bad things will happen! (NullPointerException, or a fresh SpanWriter with no buffer, depending on the situation).

SpanWriter can also now be used with a fixed statement since it has a `PinnableReference()` function.

Projects/UOContent.Tests/UOContent.Tests.csproj

@@ -3,7 +3,7 @@
         <IsPackable>false</IsPackable>
     </PropertyGroup>
     <ItemGroup>
-        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.7.1" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.9.0-preview-20201123-03" />
         <PackageReference Include="xunit" Version="2.4.1" />
         <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3" />
         <PackageReference Include="coverlet.collector" Version="1.3.0" />