Add ConPty APIs

[davidhewitt]

Closes #695.

I added the bare minimum to cover those three functions. These APIs will only work on the very newest Windows. Does anything need to be done to feature gate them?

[retep998]

There's no need to feature gate things based on Windows version because extern functions are only linked to when used. That said, if you do use these functions then your program won't run at all on older Windows versions unless you use GetProcAddress to obtain pointers to the functions instead of using these extern functions directly.

[davidhewitt]

Cool. I'd rather not use GetProcAddress but it's going to be quite a small segment of windows users able to use these currently! 😄

Do you need anything else to make this PR mergeable?

[chyyran]

1809 is now GA, so ConPTY should be available on updated Windows 10 systems.

[davidhewitt]

I've moved HPCON to wincontypes.rs and introduced a new feature as discussed. Turns out a number of the struct definitions in wincon.rs are also defined in wincontypes.h, so I've moved those at the same time.

[retep998]

When moving existing definitions to a different header, please put in pub use statements so that code still relying on the definitions in the old location is not broken.

Also when adding a new header, you also have to update the feature array in build.rs.

[davidhewitt]

Thanks - will fixup tomorrow morning :)

[davidhewitt]

Have corrected all that now. Many thanks for your patience in helping refine all the bits I originally missed. Would a separate PR be useful to extend the notes in CONTRIBUTING to make those points clearer?

[davidhewitt]

Just wondering if there's anything else you'd like to see to have this merged?

[davidhewitt]

@retep998 just pinging to say we're hoping to merge alacritty/alacritty#1762 but would like to see these definitions merged into winapi first (we're using this patch I'm providing). Please let me know if there's anything left I can do to help this PR merge.

[davidhewitt]

Thanks!