Change SentenceEncoder::encode to take &self immutably

[danieldk]

This requires changes accross the board, since CategoricalEncoder used a numberer immutably. This change adds a type parameter to the CategoricalEncoder trait that should implement the Mutability trait. There are two crate-private data structures that implement this trait:

• Immutable: wraps Numberer directly, never updates the Numberer. If a value is not in the Numberer, it returns None.
• Mutable: wraps Numberer in a RefCell. Updates the Numberer when new values are encountered.

Two corresponding type aliases are introduced: ImmutableCategoricalEncoder and MutableCategoricalEncoder. The ImmutableCategoricalEncoder returns the special value 0 when an encoding is unknown.

Throughout sticker we now only use ImmutableCategoricalEncoder, so it should always return a known number, thereby also fixing #28.

Fixes #155

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Currently a draft PR, since this still needs testing.

[danieldk]

Doing a test run now, at least there are validation losses :).

[danieldk]

Doing a test run now, at least there are validation losses :).

Works as it did before, so I converted this into a proper non-review PR. I will also test it with an existing model, just to be 100% certain that nothing breaks.