Improve performance of ignoring files (#1971)

* Determine the type of ignore pattern once per pattern.

Performing this work when checking when each file was ignored accounted
for around 4.5 of the 6 seconds that processing ignored files was taking
on a site with ~14,000 files and a small number of ignore patterns.
After this change, processing ignored files takes less than 1.5 seconds.

* Cache the normalized paths on Resource and SourceFile.

Normalizing the paths is expensive, so avoid doing it multiple
times. `Util::normalize_path` is marked as memoized, but this
is not effective as:

1. `memoize` doesn't appear to work with module functions.
2. Checking whether we have a memoized value is as expensive as
   normalizing the path.

This further drops the time it takes to process ignored files on a site
with ~14,000 files from ~1.5 seconds to ~0.6 seconds.

middleman-core/lib/middleman-core/sources.rb

@@ -8,6 +8,10 @@ module Middleman
       ::Middleman::Sources.file_cache[full_path] ||= {}
       ::Middleman::Sources.file_cache[full_path][version] ||= ::File.read(full_path)
     end
+
+    def normalized_relative_path
+      @normalized_relative_path ||= ::Middleman::Util.normalize_path relative_path.to_s
+    end
   end
 
   # Sources handle multiple on-disk collections of files which make up