[C#] Implement IEqualityComparer<Schema> and IEqualityComparer<Field>

[asfimport]

Implement IEqualityComparer and IEqualityComparer

Both are necessary to check for Schema/Field equality. 

Reporter: Prashanth Govindarajan / @pgovind

_{Note: This issue was originally created as ARROW-4666. Please see the migration documentation for further details.}

[CurtHagenlocher]

Superceded by apache/arrow-dotnet#181.