symbolizer: cache each DWARF DIE's nextOffset and nextSiblingOffset

[oremanj]

This avoids exponential growth in runtime when parsing deeply nested debug info. The scenario that would otherwise lead to exponential growth involves forEachChild() with an argument lambda that itself recurses into children. At each layer of depth, forEachChild() iterates over all indirect children (to skip them) and so does the argument lambda (to process the top-level ones and recurse on their children). So each top-level DIE is visited twice, each second-level is visited four times, each third-level one is visited eight times, and so on.