[ruby/irb] Reduce internal operations' exposure to benchmarking

(https://github.com/ruby/irb/pull/618)

* Test last value is assigned with measure enabled

* Remove unnecessary `result` variable

`Context#evaluate` always assigns the result of the evaluation to `_` so
we don't need to do it in `Irb#eval_input`.

* Move benchmarking logic into `Context#evaluate`

Current location of the benchmarking logic is too high up and includes
operations like command loading and argument transformation, which should
be excluded. So this commit moves it into `Context#evaluate` to reduce the
noise.

We don't move it further down to `Workspace#evaluate` because `Context`
is an argument of the measure block, which is not available in `Workspace`.

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