reduce export count by separating bridge and implementation

[stevenbrix]

this PR seperates the bridge and implementation and makes all implementation of interfaces internal.

prior to this pr the "implementation" served two purposes:

1. defining how to translate between C<->swift for the InterfaceWrapper.
2. providing Swift implementations for interfaces

The InterfaceWrapper has to be public so that it can be referenced across modules. However, we don't technically need to have the implementation be public. The only thing that needs to be public is the Bridge. The Bridge is what tells the wrapper how to translate between C pointers and Swift objects. The wrapper takes the contract specified by the Bridge and actually holds the VTable and Swift objects. This wrapper is passed to WinRT and is also what knows how to unwrap a pointer via checking for the ISwiftImplemented interface.

Future improvements could fold the bridge into the wrapper, but this is a good first step.

This reduces the number of exports in WinUI.dll by 7k. Since marking APIs as internal doesn't reduce exports in debug builds, future reductions would be to write types on a per file basis. This would let us mark certain details as fileprivate

FIXES WIN-915

[linear]

WIN-915 Make IMPL types internal to reduce the export count

[tristanlabelle]

Sweet!! Looking forward to see the impact on the export count!