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.

diff --cc src/librand/distributions/exponential.rs
Simple merge

diff --cc src/librand/distributions/normal.rs
Simple merge

diff --cc src/librand/lib.rs
Simple merge

diff --cc src/librustc/metadata/csearch.rs
Simple merge

diff --cc src/librustc/metadata/decoder.rs
Simple merge

diff --cc src/librustc/middle/graph.rs
Simple merge

diff --cc src/librustc/middle/ty.rs
Simple merge

diff --cc src/librustc/middle/typeck/infer/coercion.rs
Simple merge

diff --cc src/librustc/middle/typeck/infer/lub.rs
Simple merge

diff --cc src/libstd/rt/task.rs
Simple merge

diff --cc src/libsyntax/ast.rs
Simple merge

diff --cc src/libsyntax/ast_map.rs
Simple merge