Improve build process

[omershlo]

Currently the build.rs file will re-install pari library each time we cargo build. This is taking very long time and unnecessary if pari is already installed. Find a way to change build.rs to check if pari is already installed and if it does - skip installation.

[knarz]

The build process could use some overhauling in general. For Windows/macOS the library should just be statically linked and not leave any artifacts on the system.

[omershlo]

I agree. can you help ?

[amanusk]

Suggestion: PARI is a well maintained library, and has extensive installation documentation ftp://pari.math.u-bordeaux.fr/pub/pari/manuals/2.7.6/INSTALL.pdf as well as packages in at least some Linux distributions: https://packages.ubuntu.com/bionic/libpari-dev

Why not add interactions for installation, then remove the entire make process from build.rs?

I was able to compile and pass all tests without rebuilding the local library (which of course takes a lot less time)

Alternatively, add it as a git submodule, with installation instructions

[omershlo]

I saw this method in other libraries. One motivation is that in my method you don't need to update version and make adjustments for each new Pari versions. Another possible advantage is that we can make changes to Pari