Dump ancestors' methods by ls command

[pocke]

ls command has been implemented by #203.
Pry's ls displays ancestors' methods, but IRB doesn't.

This pull request makes ls command display ancestors' methods.

Example

With the following definition:

class P
  def p() end
end

class C < P
  def m1() end
end

module M
  def m2() end
end

module M2
  include M
  def m3() end
end

obj = C.new
obj.extend M2
def obj.m4() end

before

irb(main):043:0> ls obj
C.methods: m4
C#methods: m1

after

irb(main):026:0> ls obj
M#methods: m2
M2#methods: m3
C.methods: m4
P#methods: p
C#methods: m1

---

Note that it omits Object and Object's ancestors because we can assure the argument inherits Object class easily. Pry has the same behavior.

[aycabta]

Good catch!