message: Cache rust dependencies in appveyor.
diff: @@ -5,6 +5,9 @@ environment:
  - TARGET: x86_64-pc-windows-msvc
  GNUPLOT: no
  +cache:
  + - 'C:\Users\appveyor\.cargo'
  +
  install:
  - curl -sSf -o rustup-init.exe https://win.rustup.rs/
  - rustup-init.exe -y --default-host %TARGET% --default-toolchain stable
  