Instead of one catchall trait for contexts, there are now several:
* `Context` is now a "use-only" context, allowing only to retrieve variable names and call functions.
* `ContextWithMutableVariables` also allows to mutate variables.
* `ContextWithMutableFunctions` also allows to mutate functions. However this is not used anywhere at the moment. It will come in handy when the evalexpr language gets a function definition feature.
* `GetFunctionContext` also allows to retrieve functions as `Function` type. This is uncommented at the moment, as it does not have any use in the crate as of now, and likely also not with planned future features.
Additionally, this commit makes functions generic over the context type, as opposed to having a dynamic reference to a context type.
Relates to #73
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
