Fix a possible GC hole (dotnet#1890)

* Fix a possible GC hole

See issue dotnet#1886. Will need another update when the
new [RefEscapes] attribute is available, to make this
finally really safe.

Note: This removes the public AsSpan() method for
netstandard2.0, because it cannot be made safe at all there.

* Some attempt that ... doesn't work

Because Span<T> does not implement any interfaces (a ref struct can't),
this now fails to compile with the new API.

* Remove AsSpan() altogether

But rely on ValueArray implementing IReadOnlyList, which
works just fine for our use cases.

* Avoid an unnecessary boxing conversion

* Avoid unnecessary memory allocation for foreach() loops

* Add suppression for the now missing AsSpan() method

* Remove ValueArray and adjust DHTxx accordingly

* Update CompatibilitySuppressions.xml

Author: pgrawehr

src/Iot.Device.Bindings/CompatibilitySuppressions.xml

@@ -210,4 +210,25 @@
     <Right>lib/netstandard2.0/Iot.Device.Bindings.dll</Right>
     <IsBaselineSuppression>true</IsBaselineSuppression>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0001</DiagnosticId>
+    <Target>T:Iot.Device.Common.ValueArray`1</Target>
+    <Left>lib/net6.0/Iot.Device.Bindings.dll</Left>
+    <Right>lib/net6.0/Iot.Device.Bindings.dll</Right>
+    <IsBaselineSuppression>true</IsBaselineSuppression>
+  </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0001</DiagnosticId>
+    <Target>T:Iot.Device.Common.ValueArray`1</Target>
+    <Left>lib/netcoreapp3.1/Iot.Device.Bindings.dll</Left>
+    <Right>lib/netcoreapp3.1/Iot.Device.Bindings.dll</Right>
+    <IsBaselineSuppression>true</IsBaselineSuppression>
+  </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0001</DiagnosticId>
+    <Target>T:Iot.Device.Common.ValueArray`1</Target>
+    <Left>lib/netstandard2.0/Iot.Device.Bindings.dll</Left>
+    <Right>lib/netstandard2.0/Iot.Device.Bindings.dll</Right>
+    <IsBaselineSuppression>true</IsBaselineSuppression>
+  </Suppression>
 </Suppressions>