v0.7.0-preview

Several breaking changes were made to clean things up for release.

• Renamed several instructions to match the published WebAssembly specification.
  This project started over a year before the final specification was released so some of the names it used became out of date.
• ElementType member AnyFunction has been renamed to FunctionReference to align with the spec.
• Function member Type was changed from a public field to a property.
• The Type and ValueType types were renamed to WebAssemblyType and WebAssemblyValueType, respectively, to avoid conflicts with the popular System namespace.
• .NET Standard 2.0 is now the only build produced.
  This means .NET Framework consumers must be 4.7.2 or higher.
  Infrastructure is still in place to support builds as old as .NET 4.5 and .NET Standard 1.1 if a compelling use case arises, but this may eventually be removed otherwise.