Create PreppedTokenSequence class to incapsulate getting full tokens from subtokens

[hlibbabii]

The tasks for the new PreppedTokenSequence class are to encapsulate getting full tokens from subtokens (which is currently done by FullTokenIterator class) and at the same time provide transparent access to the subtokens)

Motivation:

• To get the full tokens, the user won't have to know about FullTokenIterator. This functionality can be provided by PreppedTokenSequence directly
• ModelContext class is not really needed anymore

Provisional API:

>>> prepped_token_sequence = api.bpe("getName(", "5k")
>>> prepped_tokens
['get', 'Name', '</t>', (]
>>> prepped_tokens.metadata.token_types
[SplitContainer, OpeningBracket]
>>> prepped_tokens.metadata.n_subtokens_per_token
[3, 1]
>>> prepped_tokens.full_tokens()
[['get', 'Name', '</t>'], ['(']]
>>> prepped_tokens.full_tokens(formatter=lambda s: ''.join(s))
['getName</t>', '(']