Introduce a InputSplit trait

[Stargateur]

Closes #1450

I choice to introduce a new trait to allow a transition and avoid mistake. As we can see this produce a much better code for anychar() removing one step (maybe save CPU cycles)

This also allow to introduce a nice take_one() combinator by updating #1364 (we could also think about remove anychar and have a generic take_one() combinator)

Maybe change to just introduce a SplitFirst trait

[coveralls]

Coverage increased (+0.2%) to 81.29% when pulling 0179f2f on Stargateur:split_fisrt into 02433c2 on Geal:main.

[Stargateur]

Maybe do:

/// Abstracts split_first
pub trait InputSplit: Sized {
  /// The current input type is a sequence of that `Item` type.
  ///
  /// Example: `u8` for `&[u8]` or `char` for `&str`
  type Item;

  /// Divides one input into two at an index.
  fn split_at(self, mid: usize) -> Result<(Self, Self), (Needed, Self)>;

  /// Returns the first and all the rest of the elements of the slice, or None if it is empty.
  fn split_first(self) -> Result<(Self::Item, Self), Self>;

  /// Returns the last and all the rest of the elements of the slice, or None if it is empty.
  fn split_last(self) -> Result<(Self::Item, Self), Self>;
}

Allowing consuming self. (that could allow nice implementation for VecDeque who would use a VecDeque for nom ? no idea or just an input that is costly to create)