Prevent leakage of fmt! into the compiler

We're not outright removing fmt! just yet, but this prevents it from leaking
into the compiler further (it's still turned on by default for all other code).
std: Remove usage of fmt!
extra: Remove usage of fmt!
syntax: Remove usage of fmt!
rustc: Remove usage of fmt!
rusti: Remove usage of fmt!
rustdoc: Remove usage of fmt!
rustpkg: Remove uses of fmt!
compiletest: Remove usage of fmt!
rpass: Remove usage of fmt!
cfail: Remove usage of fmt!
rfail: Remove usage of fmt!