A very simple cache. Works just like a Hash, but can optionally store values during the fetch process.

Example:

  hc = HashCache.new
  hc.fetch("foo")
  => nil
  hc.fetch("foo"){:bar} # Block gets called because 'foo' is nil
  => :bar
  hc.fetch("foo"){raise "Block won't be called because 'foo' is cached"}
  => :bar