Type-case based on literal value

[soutaro]

Steep now analyzes case-when exhaustiveness based on value equality with literal types.

# @type var x: :foo | :bar | nil | Integer
x = nil 

# @type var a: bool
a = case x
    when nil
      true    # x: nil
    when :foo
      false   # x: :foo
    when Symbol
      true    # x: :bar
    when Integer
      false   # x: Integer
    end       # And this case is exhaustive! ❗