* Removed expect_... methods and replaced them with .as_...() methods. This removes the need to import the free-standing methods every time and makes the code cleaner.
* Changed all the examples appropriately.
Implements #53
Manage sequence operators, which currently are `Chain` and `Token` on the stack without ever inserting unfinished sequence operator nodes into another node.
Relates to #44
* Rename `HashMapConfiguration` to `HashMapContext`
* Rename `EmptyConfiguration` to `EmptyContext`
* Implement `Context` for both predefined contexts
* Update tests and doctests
Relates to #22
+ Add comma token
+ Add tuple operator
+ Add tuple value type
+ Implement display for `tree::Node` and all related types
+ Add support for right-to-left chaining of operators with same precedence
The priority of chaining of operators with the same precedence remains left-to-right.
Only if two consecutive operators with the same precedence are right-to-left, they will be ordered right-to-left.
Implements #4.
