Introduce code analyzer

[TomFinley]

Like most sufficiently large codebases the ML.NET project is guilty of having acquired a set of idioms. Internally we had a code analyzer, to help catch some of the most common issues that tended to come up in PRs, but sometimes we don't do this, and need to fix issues later (e.g., #271, #442, #478). I want to migrate that analyzer to the open source repository, to hopefully automate some of this.

There are of course other things we could do with an analyzer, once we have one.

/cc @ericstj

[TomFinley]

It seems like there might be two styles of analyzers. There are analyzers for maintaining internal code consistency, quality, and style (as seen in #557, this necessarily could not possibly depend on ML.NET), as well as analyzers made to facilitate a user using ML.NET, to make certain tasks easier, or to warn when then API is potentially being misused.