Support alias in rbs prototype rb

[pocke]

This pull request makes rbs prototype rb to support alias.

Currently rbs prototype rb just ignores alias syntax and alias_method. It will output alias by this change.

For example:

# test.rb

class C
  def m() end
  alias m2 m
end

# Before
$ rbs prototype rb test.rb
class C
  def m: () -> nil
end

# after
$ rbs prototype rb test.rb
class C
  def m: () -> nil

  alias m2 m
end

[pocke]

It is not the main part of this pull request, but attr_* accept Strings as a method names. It is the same behavior with alias_method.
So I changed them to accept Strings.

class C
  attr_reader 'a'
  attr_writer 'a'
  attr_accessor 'b'
end

c = C.new

c.a = 1
c.b = 2
p c.a, c.b
# => 1
     2

f8accb0

[soutaro]

💯