Investigate adding cons support for async streams

[DavidArno]

Async streams are a core 3.0-only feature. So in order to support this, I need to experiment with whether I can use something like:

<ItemGroup Condition=" '$(TargetFramework)' == 'net45' or '$(TargetFramework)' == 'netstandard1.0'">
    <DefaultItemExcludes>$(DefaultItemExcludes);src\Succinct\Core3.0\*.cs</DefaultItemExcludes>
</ItemGroup>

to hide files that are core 3.0-only from the compiler when targeting other targets.

A couple of handy links for the csproj stuff for my reference:
https://markheath.net/post/csproj-conditional-references
https://stackoverflow.com/questions/43173811/how-do-i-exclude-files-folders-from-a-net-core-standard-project