Omit the results evaluated at assignment if they are too long.

The behavior of ECHO_ON_ASSIGNMENT being on by default is hard to understand,
so I change it to off by default. Instead, we turn OMIT_ON_ASSIGNMENT on by
default. The result is displayed on assignment, but it will always be short
and within one line of the screen.

https://github.com/ruby/irb/commit/c5ea79d5ce