rustc: Switch tuple structs to have private fields

This is a continuation of the work done in #13184 to make struct fields private
by default. This commit finishes RFC 4 by making all tuple structs have private
fields by default. Note that enum variants are not affected.

A tuple struct having a private field means that it cannot be matched on in a
pattern match (both refutable and irrefutable), and it also cannot have a value
specified to be constructed. Similarly to private fields, switching the type of
a private field in a tuple struct should be able to be done in a backwards
compatible way.

The one snag that I ran into which wasn't mentioned in the RFC is that this
commit also forbids taking the value of a tuple struct constructor. For example,
this code now fails to compile:

    mod a {
        pub struct A(int);
    }

    let a: fn(int) -> a::A = a::A; //~ ERROR: first field is private

Although no fields are bound in this example, it exposes implementation details
through the type itself. For this reason, taking the value of a struct
constructor with private fields is forbidden (outside the containing module).

RFC: 0004-private-fields
Switch some tuple structs to pub fields

This commit deals with the fallout of the previous change by making tuples
structs have public fields where necessary (now that the fields are private by
default).
auto merge of #13237 : alexcrichton/rust/private-tuple-structs, r=brson

This is the final commit need to implement [RFC #4](https://github.com/rust-lang/rfcs/blob/master/active/0004-private-fields.md), it makes all tuple struct fields private by default, overridable with the `pub` keyword.

I'll note one divergence from the original RFC which is outlined in the first commit.