Option for linking executables statically

[judah]

It will make rebuilds take up significantly less space, and possibly take less time too. (We might be able to avoid the "dynamic-too" and "ar" invocations if we're only building shared libraries with "-shared".)

We'll want to add a linkArtifact analogue of copyArtifact, and link all dylibs in one location (similar to the workaround that Cabal/Nix/etc use for macOS Sierra).

• Make dynamic linking the default
• Allow static linking as another option
• Work around the macOS Sierra linking issue

[simonmichael]

Upvoting because I think I'm seeing "the macOS Sierra linking issue" (pier build, copy executable to ~/.local/bin, try to run it and get a dyld error, run it from the original location and it works, and now so does the one in ~/.local/bin.)