Formula API

[storopoli]

This PR implements the @formula API to TuringGLM, mostly copied from StatsModels.jl and MixedModels.jl.

Note that we are excluding the intercept in the classical StatsModels.jl model matrix construction.

One of #2.

The PR is broken in Julia nighly because of LibTask.jl issues in Julia 1.7

[codecov]

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[storopoli]

Ok this PR is getting really big. But the main @formula API is done. Please, whevener you can @rikhuijzer could you review it?