[SharedCache] Optimize ReadExportNode

[bdash]

ReadExportNode is called frequently during the initial load of the shared cache and impacts how long it takes for parts of the shared cache UI to be functional. 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.