Support decoding with ranges to avoid a copy

[jerinphilip]

For browsermt/bergamot-translator#202, it was suggested that a view for a vector/range backed by a binary representaion be substituted to eliminate conversions between vector and binary representation.

However, decodeWithByteRanges assumes a vector to be provided:

marian-dev/src/data/sentencepiece_vocab.cpp

Lines 259 to 281 in 62bac85

	void decodeWithByteRanges(const Words& sentence,
	std::string &decoded,
	std::vector<string_view> &byteRanges,
	bool ignoreEOS) const override {
	sentencepiece::SentencePieceText spt;
	std::vector<int> spmSentence;
	spmSentence.reserve(sentence.size());
	for(auto&& word : sentence)
	spmSentence.push_back(word.toWordIndex());
	spm_->Decode(spmSentence, &spt);
	decoded = spt.text(); // Creates copy of string.
	string_view decoded_view(decoded);
	for(auto piece : spt.pieces()) {
	string_view byteRange = decoded_view.substr(piece.begin(), piece.end() - piece.begin());
	byteRanges.push_back(byteRange);
	}
	if(ignoreEOS){
	byteRanges.pop_back();
	}
	}

We need to provide something that can work with ranges/iterators instead to avoid the additional copy. Since this is a function only used by bergamot we need not worry about breaking any backwards compatibility. Consistency with the remaining may still be a concern, in which case we can provide an overload which internally calls the range-based method.