[SharedCache] Introduce a SymbolInfo type

[bdash]

Previously SharedCache used a mixture of pair<uint64_t, pair<BNSymbolType, std::string>> and Ref<Symbol> to represent a symbol. The former is hard to understand due to the nesting and lack of names. The latter is expensive to create and its getters are expensive due to the multiple allocations + strlen calls performed when converting between core's string representation and std::string.

SymbolInfo wraps the same data that the nested pairs contained in a friendly struct with named fields. Beyond that it, it exposes an AsSymbol function to construct a Ref<Symbol> where that is what is required.

Code within SharedCache is updated to work with SymbolInfo until it needs to call into a core API that requires a Symbol.

This change is only a small performance improvement at the moment. It's more noticable once SharedCache no longer copies so much of its state (#6393).

[bdash]

I'll have to look at whether this is still worth doing given #6197. It may not be necessary.

[bdash]

Constructing a Symbol is still a little more expensive than I'd like, but the changes on the test_api_string_ref branch make the Symbol getters significantly more efficient. That's a better path forward than introducing a custom type.