Rollup merge of #22311 - lfairy:consistent-fmt, r=alexcrichton

 This brings it in line with its namesake in `std::io`.

[breaking-change]

r? @aturon

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

diff --cc src/libcore/fmt/float.rs
Simple merge

diff --cc src/libcore/fmt/mod.rs
Simple merge

diff --cc src/libstd/io/mod.rs
Simple merge

diff --cc src/libstd/old_io/mod.rs
Simple merge

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