This replaces cargo-fmt with rustfmt with --skip-children which should
allow us to format code without running into rust-lang/rustfmt#3930.
This also bumps up the version of rustfmt used to a more recent one.
This commit adds an rustfmt.toml for using for **new** code.
Old code should continut to use old style, until we put automated
style checks in place.
See
https://internals.rust-lang.org/t/running-rustfmt-on-rust-lang-rust-and-other-rust-lang-repositories/8732/81
for the reason why we deviate from the default formatting. The TL;DR
is that currently compiler uses a pretty condensed style of code, and
default settings both create a huge diff and inflate the number of
lines. use_small_heuristics=Max fixes that.
version=Two is required for bug-fixes, which technically can't be made
to the stable first version
