Revert v2 line length change as discussed in #670

It was clearly a mistake to accept the default formatting change in v2,
and now there's no ideal choice. Either we revert the change and break
some projects twice, or we keep the change and break some projects once.
Given the report comes from Kubernetes, which has a relevant community
and code size, we'll revert it. At the same time, to simplify the life
of those that already started migrating towards the v3 behavior, a new
FutureLineWrap function is being introduced to trivially preserve the
new behavior where desired.

The v3 branch is not affected by this, and will retain the default
non-wrapping behavior. It will also be changed soon to support per
arbitrary line-wrapping for individual encoding operations.

Thanks to everyone who offered code and ideas, and apologies for
the trouble.