Preserve original API case

[kennykerr]

Originally, the winrt-rs crate transformed API names to appear more idiomatic in Rust. However with the addition of Win32 and COM APIs in the windows-rs crate, it became increasingly difficult to continue to provide that transformation as many Win32 and COM APIs do not lend themselves to this kind of transformation. Increasingly, this has become a tax for a number of reasons.

• There are a long list of suboptimal snake_case conversions that cannot easily be handled. (Suboptimal snake_case conversion in some cases #112)
• The conversion is lossy, making it difficult to support API authoring in Rust. (Defining and Implementing Win32/COM/WinRT types in Rust #81)
• It is much harder to port Windows code from other languages.
• It is much harder to find documentation for Windows APIs when the names aren't searchable.
• This project attempts to treat all Windows APIs alike, and having different conventions for Win32 and WinRT is really confusing.

This is going to be a little painful for existing code using the windows crate - creating this PR was a bit tedious - but in the long run I believe this will provide the best experience, and the sooner the better. Now that my focus is shifting to #81 this is really essential.