Cleanup/reorganize header field constants

[thilo-schmitt]

We now have a huge list of constatns (constexprs, I guess) used for describing the offset and length of header fields. This is not too pleasant to read. We might be able to remodel/reorganize that into something more descriptive and less loose-coupled-lists style.

For example, maybe something like the following?

enum class header_field_name {
  v7_ustar_name,
  //...
  ustar_magic,
  //...
};
struct header_field_descriptor {
  unsigned offset;
  unsigned length;
};
const std::map<header_field_name, header_field_descriptor> tar_headers = {
  {header_field_name::v7_ustar_name, {0u, 100u}},
  //...
  {header_field_name::ustar_magic, {257u, 6u}},
  //...
};