Refine signature of Numeric#step and add type of Enumerator::ArithmeticSequence

[ksss]

There are several mistakes in the type of the #step method.

Problem

Mistakes in block variable type

The one of Integer#step overload is here.

(Numeric limit, ?Integer step) { (Integer) -> void } -> void

But, i is Float in following code.

1.step(3.0, 1) { |i| p i }
# 1.0
# 2.0
# 3.0

Return type without block

In ruby, the method that returns Enumerator::ArithmeticSequence but signature returns Enumerator type.

1.step #=> Enumerator::ArithmeticSequence

Documentation

When hovering over 1.step in Steep, I cannot read the rdoc in LSP.

Solution

Add type of Enumerator::ArithmeticSequence

Return Enumerator::ArithmeticSequence to match the actual behavior.
Enumerator::ArithmeticSequence introduced from ruby v2.6.
We will be able to type Enumerator::ArithmeticSequence specific methods.

1.step.begin #=> Numeric

Notes

The Enumerator::ArithmeticSequence holds several values.

a = 1.step(3r, 1.0)
a.begin #=> 1
a.step #=> 1.0
a.end #=> 3r

However, the Ruby specification is more complex than expected.

1.step.end #=> nil
(..3).step(1).begin #=> nil
1.step(3.0, 1r) { p it } #=> 1.0, 2.0, 3.0

The use of Enumerator::ArithmeticSequence is limited, and since there haven't been any requests for it so far, I don't think there's a need to make it overly complex.

Unify signatures to Numeric#step

It is possible to copy the documentation, but if the goal is to keep it simple, unifying it under Numeric#step would make it easier to manage.