refactor all to async/await (#617)

Co-authored-by: Danny Browning <danny.browning@protectwise.com>
Co-authored-by: Daniel Eades <danieleades@hotmail.com>

.appveyor.yml

@@ -8,7 +8,7 @@ environment:
     MINGW_PATH: 'C:\MinGW\bin'
 install:
   - curl -sSf -o rustup-init.exe https://win.rustup.rs/
-  - rustup-init.exe -y --default-host %TARGET%
+  - rustup-init.exe -y --default-toolchain nightly --default-host %TARGET%
   - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin
   - if defined MINGW_PATH set PATH=%PATH%;%MINGW_PATH%
   - rustc -vV