tar: Refactor tar writer, add a struct to represent tar headers

[euanh]

Motivation

The tar writer is able to write a single file into a tar archive. This is enough to build a container image around a single executable, but adding resource bundles (issue #48) requires the ability to write multiple files, directories and possibly symlinks into the archive. This commit begins to refactor the tar writer to support this. A future commit will add interoperability testing to ensure that more complex archives can be unpacked correctly.

Modifications

• Add a TarHeader struct representing a standard tar archive member
• Convert type flags to an enum
• Wrap fields in a namespacing enum

Result

No functional change. Tar writer is more modular, easier to test and extend.

Test Plan

Existing tests continue to pass.