Enable an efficent way of parsing only the compile unit DIE

[philipc]

In order to parse the compile unit DIE, we currently need to parse all abbreviations, and allocate storage for them. It would be nice to have a way of parsing only the abbreviation for the compile unit (no allocation needed).

For example, this would be useful in addr2line when running without the '-f' option.

One way of doing this would be to change Abbreviations to be a trait. This would also allow alternative implementations that used a storage method of the user's choice (although maybe that wouldn't be beneficial unless Abbreviation was also a trait.)

Not sure if this is worth the additional complexity though.

[philipc]

Another option would be to make Abbreviations parse lazily.