builds on windows are broken and subpar in general

[Trolldemorted]

During a train ride through rural Europe I had some trouble to use z3-sys due to several reasons, so I thought I should open an issue and PR to streamline the overall user experience of developers.

I have identified a few problems:

• Building z3-sys requires a bespoken clang installation. I installed clang via the visual studio installer and it works nice in cmd and powershell, but your current build process does not find it and fails.
• Building z3-sys requires a clang installation at all, despite being a shallow wrapper around the native library. I know having bindgen in your build script has its advantages, but it should not be in the default case, but rather hidden behind a feature.
• The installation documentation is incomplete and partially misleading.
• Using precompiled Z3 binaries is not supported. Using z3-sys on weak/old systems imposes a long initial build time (sad old laptop noises).
• Several users were unable to build z3-sys (Can't build z3-sys on windows #183, z3-sys fails to build on Windows? #87) due to the issues identified above.

I'll open a PR that tries to address/amend some of these.