Don't call `Kernel#require` in hot loop

[casperisfine]

Ref: https://bugs.ruby-lang.org/issues/20641

Even without the reference bug, require 'date' isn't cheap.

require "benchmark/ips"
require "yaml"
require "date"

100.times do |i|
  $LOAD_PATH.unshift("/tmp/does/not/exist/#{i}")
end
payload = 100.times.map { Date.today }.to_yaml

Benchmark.ips do |x|
  x.report("100 dates") { YAML.unsafe_load(payload) }
end

Before:

$ ruby /tmp/bench-yaml.rb
ruby 3.2.2 (2023-03-30 revision e51014f9c0) [arm64-darwin22]
Warming up --------------------------------------
           100 dates   416.000 i/100ms
Calculating -------------------------------------
           100 dates      4.309k (± 1.2%) i/s -     21.632k in   5.021003s

After:

$ ruby -Ilib /tmp/bench-yaml.rb
ruby 3.2.2 (2023-03-30 revision e51014f9c0) [arm64-darwin22]
Warming up --------------------------------------
           100 dates   601.000 i/100ms
Calculating -------------------------------------
           100 dates      5.993k (± 1.8%) i/s -     30.050k in   5.016079s

@tenderlove