Windows CI using AppVeyor

New configuration file appveyor.yml
- Downloads and installs latest Julia binary, uses it for dependency libraries
- Also uses MSYS from bundled Git installation

Build script contrib/windows/msys_build.sh
- Downloads MinGW, LLVM, make, etc
- Sets up Make.user to use as many downloaded dependencies as possible
Use msys with a different mingw build, cleaner method for silent downloads

Clean up the MSVC side of the build script

Rehost cut-down MSVC LLVM binary
Update for removal of readline and renaming of julia exe

Remove incompatible pre-installed mingw from AppVeyor path

Force tests to run in parallel

Use compile and ar-lib from libuv for MSVC