Show real LoadError on helpers require

When helper try to require missing file rails will throw exception about
missing helper.

  # app/helpers/my_helper.rb

  require 'missing'

  module MyHelper
  end

And when we try do load helper

  class ApplicationController
    helper :my
  end

Rails will throw exception. This is wrong because there is a helper
file.

  Missing helper file helpers/my_helper.rb

Now when helper try to require non-existed file rails will throw proper
exception.

  No such file to load -- missing
Speed up AS::Inflector.underscore

Benchmark:
          user     system      total        real
old   6.090000   0.120000   6.210000 (  6.202039)
new   5.930000   0.110000   6.040000 (  6.042022)
Merge pull request #10642 from LTe/invalid-load-error

Show real LoadError on helpers require
Speed up String#truncate

Benchmark:
          user     system      total        real
old   1.550000   0.040000   1.590000 (  1.585866)
new   1.250000   0.040000   1.290000 (  1.287693)
Speed up Array#split

Benchmark:
          user     system      total        real
old   0.510000   0.000000   0.510000 (  0.506749)
new   0.330000   0.000000   0.330000 (  0.336187)
Simplify Array#in_groups_of code