Switched from winapi to windows-rs

[ancwrd1]

This PR resolves #75. Important bullet points:

• I have used windows-sys crate which is roughly equivalent to winapi. The higher-level windows crate doesn't currently work well with the functions returning HRESULT and suffers from occasional breaking changes in the APIs
• The MSRV is 1.40 due to added non_exhaustive pattern which replaces the manual one on several enums
• The code was formatted with rustfmt and updated to 2018 edition
• Fixed a few minor clippy warnings
• Tested with both -msvc and -gnu targets
• Mostly translates one to one with a few tweaks around mutability and types

[steffengy]

Updated and revived appveyor tests on master and checked that this builds through CI given with 1.40 as min version: https://ci.appveyor.com/project/steffengy/schannel-rs/builds/43467207

Build times seem the same, so no issue on that front. Need to still take a look at the code and check some compatiblity things.

[steffengy]

Thanks