auto merge of #14414 : richo/rust/features/nerf_unused_string_fns, r=alexcrichton

This should block on #14323

diff --cc src/doc/tutorial.md
Simple merge

diff --cc src/libcollections/hashmap.rs
Simple merge

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

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

diff --cc src/libserialize/json.rs
Simple merge

diff --cc src/libstd/str.rs
Simple merge

diff --cc src/libsyntax/ext/expand.rs
Simple merge

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

diff --cc src/libsyntax/parse/parser.rs
Simple merge