Performance and extract work

[mmastrac]

• Add support for extract in pattern definitions, allowing the extract in
  %{name:alias:extract} to be retrieved from the pattern.
• Add Pattern::pattern() to Matches to get the pattern that was used to
  match this Matches instance.
• Reduced the scope of inline pattern definitions to the current pattern only
  (before 2.2.0 they were added globally, in 2.2.0 they were available to all
  nested patterns, and in 2.3.0 they are only available to the current
  pattern).
• Grok::compile() is now &self instead of &mut self.
• Grok::with_default_patterns() now uses Cow for the built-in patterns and
  allocates significantly less per Grok instance (as well as being much
  faster).
• Built-in patterns are available individually for reference in the new
  patterns module.