Introduce RecordVisitor. This is used for different clients that want to
extract information out of RecordSlice types.
The first and immediate use case is for serializing symbol information
into TBD files.
> Add support for reading binary Mach-o dynamic libraries. It uses
libObject APIs for extracting information relevant to TAPI and tbd
files. This includes but is not limited to load commands encode data
like install names, current/compat versions, and symbols.
This originally broke because DylibReader uses Object and Object depends
on TextAPI. Breaking this up in a nested library prevents this cycle.
