Repeatedly load sig directory from loaded gems

[tompng]

Loads rbs defined in gem sig directory.
Adds ReplTypeCompletor.reload_rbs, which should be called from IRB in InputMethod#gets

classIRB::TypeCompletordefreloadReplTypeCompletor.reload_rbsrescuenilendendclassIRB::ReadlineInputMethod < StdioInputMethoddefgets@completor.reload
...
endend

ReplTypeCompletor reloads rbs when additional gem is loaded (by checking $LOADED_FEATURES difference)

# loads sig defined in required gems
irb(main):001> Prism.parse('1').value.statements.b
'1').value.statements.body
irb(main):002> require 'rmagick'
irb(main):003> # Automatically loads rmagic/sig/*.rbs in another thread
irb(main):003> pix = Magick::Pixel.new; pix.red.ab
pix.red.abs pix.red.abs2