[SharedCache] Optimize `ReadExportNode`

`ReadExportNode` is called a lot during the initial load of the shared
cache and thus impacts how long it takes for the UI to become
responsive. This is a collection of optimizations that cut the time
spent within `ReadExportNode` by 50%:

1. Pass iterators to `ReadExportNode` rather than a `DataBuffer` + offset.
   The lack of inlining in `DataBuffer`'s `operator[]` kills performance.
   Ideally this would have used `std::span`, but that would require
   bumping the minimum C++ version to C++20.
2. Add `MMappedFileAccessor::ReadSpan` so that `ReadExportNode` can
   operate directly on the mapped data without first copying it.
3. Removes a call to `GetAnalysisFunctionsForAddress` whose result was
   unused.
4. Use `std::find` to find the nul at the end of strings rather than
   assembling the string a character at a time. This avoids repeatedly
   growing the string.
5. Avoid the usual `Symbol` constructor in favor of `BNCreateSymbol`.
   The `Symbol` constructor has over head in two forms:
   1. It has a `NameSpace` as an argument. Creating / destroying this
      allocates and deallocates memory We could create a single
      instance and reuse it for all calls, but...
   2. The constructor copies all fields of the `NameSpace` to the heap
      before calling `BNCreateSymbol` and then deallocates them
      afterwards. This seems unnecessary, and adds a non-trivial amount
      of overhead.

   This can go back to directly constructing the `Symbol` once the
   constructors are improved.

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