Revert "Put single-expression functions on a single line"

[mgeisler]

This reverts commit 4ce32d6.

Changing the default formatting using an unstable rustfmt feature
was a bad idea: it means that people who have auto-formatting turned
on (like me) will see the formatting flip-flop when switching between
stable and nightly.

The problem is that the unstable fn_single_line option only takes
effect on nightly, so when rustfmt runs on stable, it will add lots
of spurious changes.

The imports_granularity option does not have this problem: rustfmt
will not modify imports by default and so nothing happens when it is
run on stable.