Load IRB lazily

[pocke]

This PR makes loading IRB lazy. It improves startup time.

Problem

Byebug always loads IRB, but IRB is necessary only for irb command. It makes startup time slower unnecessarily for non-IRB users.

Solution

Let's load IRB lazily.

Benchmark

About 55ms (50%) faster.

before

$ RUBYLIB=lib ruby -rbenchmark -e 'p Benchmark.realtime{require "byebug/core"};'
0.11415267689153552

after

$ RUBYLIB=lib ruby -rbenchmark -e 'p Benchmark.realtime{require "byebug/core"};'
0.059115791926160455