PreprocessingMetadata enhancement

[hlibbabii]

• Rename PreprocessingMetadata -> PreppedTokenMetadata
• Represent word_boundaries field as a list of the number of subtoken in each token, e.g
  [1, 3, 1, 2] instead of [0, 1, 4, 5, 7]
• Remove non-processible tokens filed. Return non-processible tokens as a separate object
• Provide a method for returning the metadata for the last tokens:

>>> metadata.for_last_tokens(n: int)

[hlibbabii]

This enhancement is useful for easier implementation of the calculation of the context statistics in giganticode-langmodels