Support type inference in conditional types ('infer')

[mjambon]

The following isn't supported at the moment:

type Elt<Container> = Container extends (infer X)[] ? X : never;

What's not supported is the infer X in the pattern (infer X)[].

Documentation: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-8.html#type-inference-in-conditional-types

[resolritter]

Now that #114 has landed in master, I am planning to work on this one