This PR restructures the vocabulary.
* String based look-ups are removed. Vocabulary is changed from a map to vector. (#141832)
* Grouped all the vocabulary related methods under a single class - `ir2vec::Vocabulary`. This replaces `IR2VecVocabResult`.
* `ir2vec::Vocabulary` effectively abstracts out the _layout_ and other internal details of the vector structure. Exposes necessary APIs for accessing the Vocabulary.
These changes ensure that _all_ known opcodes and types are present in the vocabulary. We have retained the original operands. This can be extended going forward.
(Tracking issue - #141817)
e86bd05bdc