Faster Pathname FileUtils methods

Currently when calling any of the "FileUtils" methods on pathname `require` is called every time even though that library might already be loaded. This is slow:

We can speed it up by either checking first if the constant is already defined, or by using autoload.

Using defined speeds up the action by about 300x and using autoload is about twice as fast as that (600x faster than current require method).

I'm proposing we use autoload:

```ruby
require 'benchmark/ips'

Benchmark.ips do |x|
  autoload(:FileUtils, "fileutils")
  x.report("require") { require 'fileutils' }
  x.report("defined") { require 'fileutils' unless defined?(FileUtils) }
  x.report("autoload") { FileUtils }

  x.compare!
end

# Warming up --------------------------------------
#              require     3.624k i/100ms
#              defined     1.465M i/100ms
#             autoload     2.320M i/100ms
# Calculating -------------------------------------
#              require     36.282k (± 2.4%) i/s -    184.824k in   5.097153s
#              defined     14.539M (± 2.0%) i/s -     73.260M in   5.041161s
#             autoload     23.100M (± 1.9%) i/s -    115.993M in   5.023271s

# Comparison:
#             autoload: 23099779.2 i/s
#              defined: 14538544.9 i/s - 1.59x  (± 0.00) slower
#              require:    36282.3 i/s - 636.67x  (± 0.00) slower
```

Because this autoload is scoped to Pathname it will not change the behavior of existing programs that are not expecting FileUtils to be loaded yet:

```
ruby -rpathname -e "class Pathname; autoload(:FileUtils, 'fileutils'); end; puts FileUtils.exist?"
Traceback (most recent call last):
-e:1:in `<main>': uninitialized constant FileUtils (NameError)
```

ext/pathname/lib/pathname.rb

@@ -575,12 +575,13 @@ end
 
 
 class Pathname    # * FileUtils *
+  autoload(:FileUtils, 'fileutils')
+
   # Creates a full path, including any intermediate directories that don't yet
   # exist.
   #
   # See FileUtils.mkpath and FileUtils.mkdir_p
   def mkpath
-    require 'fileutils'
     FileUtils.mkpath(@path)
     nil
   end
@@ -591,7 +592,6 @@ class Pathname    # * FileUtils *
   def rmtree
     # The name "rmtree" is borrowed from File::Path of Perl.
     # File::Path provides "mkpath" and "rmtree".
-    require 'fileutils'
     FileUtils.rm_r(@path)
     nil
   end