Require rbs outside of thread to avoid deadlock

[tompng]

IRB's debug command stops all threads. When thread which is running require 'rbs' stops, all require will be blocked forever because require has exclusive control.
In IRB, pressing TAB will start require 'rdoc' but it will deadlock if debug mode started before require 'rbs' completes.

To avoid it, all require should be done in main thread.
This will make IRB startup time slow about 43%.

# Benchmarks

PTY.spawn('irb', '--regexp-completor').first.getc
# processing time: 0.168581s

PTY.spawn('irb', '--type-completor').first.getc # main branch
# processing time: 0.189315s

PTY.spawn('irb', '--type-completor').first.getc # this branch
# processing time: 0.271634s

[st0012]

I think this slowdown is acceptable. But I wonder if using autoload could be a better solution? AFAIK, debug doesn't interfere code autoloading.

[tompng]

Autoload will delay loading until its needed. But loading *.rbs should start as soon as possible to make the chance of type information ready before user enters 1.times.m which needs type to autocomplete.