Add as_mut, as_ref, as_inner, as_mut_inner, and into_inner for LocatedElement

[chipsenkbeil]

Inspired by Option<T>, provide support for exposing the inner value of a LocatedElement.

This involves renaming the interior from element to inner and then providing these four methods, which will be useful in #12 and all of the graph-based issues where I need references to elements.

pub fn as_ref(&self) -> LE<&T>;
pub fn as_mut(&mut self) -> LE<&mut T>;
pub fn as_inner(&self) -> &T;
pub fn as_mut_inner(&mut self) -> &mut T;
pub fn into_inner(self) -> T;