lookup() and lookup_mut() lifetime enhancements.
lookup() and lookup_mut() lifetime enhancements.

Rationale:

- The path has nothing to do with the result.
- The path has no need to live as long as the Value/self.
- In some cases it can be hard to actually build a path that meets
  the same lifetime requirements as the Value or String slice result.

GuillaumeGomez/toml-rs,alexcrichton/toml-rs