llama-server : implement universal assisted decoding

[g2mt]

This pull request implements universal assisted decoding in llama-server. This is a method for performing speculative decoding given a draft model whose tokenizer is incompatible with the main model, by decoding and reencoding the generated text between the two models.

It currently works, but some improvements can be made.

• Token healing can be done to fix any weirdness that may occur if the draft model generates tokens that doesn't lie on a word boundary (not sure how much this affects performance).
• The translation process can be cached to improve sampling time, however it might require substantial refactoring.

[jukofyork]

This looks really interesting! It's surprising how much crossover there is between many models' tokenisers.