Type inference for structs and type checking for dot/remote

[josevalim]

With this pull request, we now warn:

• expr.field when expr may not be a map
• expr.call() when expr may not be an atom
• expr.call(...) when expr may not be an atom
• &expr.foo/1 when expr may not be an atom

Furthermore, we lay down the ground work for checking undefined and deprecation warnings across unions. For example, if you write this code:

mod = if something?, do: Foo, else: Bar
mod.some_function()

In the future, it will warn if any of Foo OR Bar do not define the relevant function.

Finally, we improve pretty printing of maps and aliases.

[josevalim]

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