Ruby 2.5: Add String#delete_prefix and String#delete_prefix!

[ChrisBr]

See feature https://bugs.ruby-lang.org/issues/12694

#4876

[ChrisBr]

Please note that the last test in test_delete_prefix! currently fails:

    s = S("ax")
    o = Struct.new(:s).new(s)
    def o.to_str
      s.freeze
      "a"
    end
    assert_raise_with_message(RuntimeError, /frozen/) {s.delete_prefix!(o)}

I'm wondering what I do wrong here as I do a frozen check in delete_prefix!
cc @enebo @headius

[ChrisBr]

@enebo @headius fixed the failing test. The problem was that the modify_check() was called before the freeze call was executed. I moved the modifyCheck() now one line down which fixed it.