Create an internal IR for DWARF info

[ayrtonm]

The EntryRef methods in dwarf_entry.rs duplicate some code since the various input sources provide different DWARF function and variable entry attributes. We could deduplicate quite a bit by creating an internal IR for DWARF function and variable entries.

Essentially these structs would have Option<T> fields for the entry attributes. We may possibly want to use a different enum with three variants to distinguish between input sources that cannot provide a certain attribute and those that can, but did not provide it. Then the parsed input data would be transformed into this IR (instead of the current AnvillData, GhidraData, etc...) and we could use a single function to update the DWARF function entries and another for variables. This wouldn't change the behavior of this tool, but would be good to have before adding any more input sources.