Fix some compiler errors that can occur with specific project setups (Memory references)

[sbeca]

While testing Polyfill in a few different projects, I hit a few compiler errors which this PR attempts to resolve.

First, in a netstandard2.0 project that references System.Threading.Tasks.Extensions but doesn't reference System.Memory, I saw the following errors:

/Users/scott/.nuget/packages/polyfill/1.32.0/contentFiles/cs/netstandard2.0/Polyfill/PolyfillExtensions_TextWriter.cs(8,14): error CS0234: The type or namespace name 'Buffers' does not exist in the namespace 'System' (are you missing an assembly reference?)
/Users/scott/.nuget/packages/polyfill/1.32.0/contentFiles/cs/netstandard2.0/Polyfill/PolyfillExtensions_Stream.cs(33,9): error CS0246: The type or namespace name 'Memory<>' could not be found (are you missing a using directive or an assembly reference?)
/Users/scott/.nuget/packages/polyfill/1.32.0/contentFiles/cs/netstandard2.0/Polyfill/PolyfillExtensions_TextReader.cs(34,9): error CS0246: The type or namespace name 'Memory<>' could not be found (are you missing a using directive or an assembly reference?)
/Users/scott/.nuget/packages/polyfill/1.32.0/contentFiles/cs/netstandard2.0/Polyfill/PolyfillExtensions_Stream.cs(56,9): error CS0246: The type or namespace name 'ReadOnlyMemory<>' could not be found (are you missing a using directive or an assembly reference?)
/Users/scott/.nuget/packages/polyfill/1.32.0/contentFiles/cs/netstandard2.0/Polyfill/PolyfillExtensions_TextWriter.cs(29,9): error CS0246: The type or namespace name 'ReadOnlyMemory<>' could not be found (are you missing a using directive or an assembly reference?)
/Users/scott/.nuget/packages/polyfill/1.32.0/contentFiles/cs/netstandard2.0/Polyfill/PolyfillExtensions_TextWriter.cs(55,9): error CS0246: The type or namespace name 'ReadOnlyMemory<>' could not be found (are you missing a using directive or an assembly reference?)
/Users/scott/.nuget/packages/polyfill/1.32.0/contentFiles/cs/netstandard2.0/Polyfill/PolyfillExtensions_TextWriter.cs(76,9): error CS0246: The type or namespace name 'ReadOnlySpan<>' could not be found (are you missing a using directive or an assembly reference?)
/Users/scott/.nuget/packages/polyfill/1.32.0/contentFiles/cs/netstandard2.0/Polyfill/PolyfillExtensions_TextWriter.cs(99,9): error CS0246: The type or namespace name 'ReadOnlySpan<>' could not be found (are you missing a using directive or an assembly reference?)

This was fixed in this PR by adding the MEMORYREFERENCED define in a few extra places.

Then, in another project that defined its own Debug class, I saw the following errors:

/Users/scott/.nuget/packages/polyfill/1.32.0/contentFiles/cs/netstandard2.0/Polyfill/Nullability/NullabilityInfoContext.cs(507,13): error CS0104: 'Debug' is an ambiguous reference between 'MBT.Utilities.Debug' and 'System.Diagnostics.Debug'
/Users/scott/.nuget/packages/polyfill/1.32.0/contentFiles/cs/netstandard2.0/Polyfill/Nullability/NullabilityInfoContext.cs(541,13): error CS0104: 'Debug' is an ambiguous reference between 'MBT.Utilities.Debug' and 'System.Diagnostics.Debug'

This was solved by adding some explicit using Debug = System.Diagnostics.Debug; lines in Polyfill.

EDIT: Second issue/fix extracted into separate PR #122.

[SimonCropp]

can you add a new project that replicates this scenario. so the changes can be verified

[sbeca]

Yep, I can look into doing that

[sbeca]

@SimonCropp: I have added a new project (ConsumeTasksWithNoMemory.csproj) and a new class (Debug.cs) that replicates these 2 issues. They both fail to compile when included in main but compile fine with the changes in this PR.

[SimonCropp]

can u split the Debug change into a different PR

[sbeca]

Sorry about the delay, I have now split the Debug change out into PR #122

[SimonCropp]

thanks. will deploy now. pending nuget updating its cache