Disable Zeitwerk eager loading to improve performance

[hidakatsuya]

# lib/rexer.rb
puts Benchmark::CAPTION
puts Benchmark.measure {
  loader = Zeitwerk::Loader.for_gem
  loader.setup
  loader.eager_load
}

$ rex state
      user     system      total        real
  0.225180   0.038047   0.263227 (  0.263247)

puts Benchmark::CAPTION
puts Benchmark.measure {
  loader = Zeitwerk::Loader.for_gem
  loader.setup
-  loader.eager_load
}

$ rex state
      user     system      total        real
  0.001415   0.000198   0.001613 (  0.001612)

For example, rex state will be about 30% faster.

v0.10.0

$ time rex state
...
real	0m0.470s
user	0m0.325s
sys	0m0.146s

This pull request:

$ time rex state
...
real	0m0.340s
user	0m0.252s
sys	0m0.089s