Reduce semantics of proxy_invoke

[mingxwa]

Why

Before this change, each polymorphic call performed with proxy accessibility API will cause overload resolution twice. Although it does not affect code generation, it introduces unnecessary compile-time dependency.

Changes

• Added overload type O to each overload of proxy_invoke.
• Updated implementation of the dispatch types accordingly.
• Updated documentation and sample code accordingly.

Notes

• Since this change involves API change, the minor version of the library should be updated later.