Initialize Tokenizer and simplify str_lookup prototype

[pagakarthik]

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• new method to initialize tokenizer with a given vocab_size
• removed voacb_size from the arguments of build_tokenizer
• applied the changes in run.c, runq.c, test.c
• pass the tokenizer object to str_lookup
• helps to easily follow - "oh! we will be checking for the string in the Tokenizer"
• makes it easy for future changes to sentencepiece code <I couldn't follow the TODO, so start with something simpe=le>
• passes: unit test (test.c)
• passes: integration test (./run stories15M.bin on Mackbook M1>