Replace tuple of infer vars for upvar_tys with single infer var This commit allows us to decide the number of captures required after completing capture ananysis, which is required as part of implementing RFC-2229. closes https://github.com/rust-lang/project-rfc-2229/issues/4 r? `@nikomatsakis`
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.