trailer: make trailer_info struct private

trailer: make trailer_info struct private

In 13211ae23f (trailer: separate public from internal portion of
trailer_iterator, 2023-09-09) we moved trailer_info behind an anonymous
struct to discourage use by trailer.h API users. However it still left
open the possibility of external use of trailer_info itself. Now that
there are no external users of trailer_info, we can make this struct
private.

Make this struct private by putting its definition inside trailer.c.
This has two benefits:

  (1) it makes the surface area of the public facing
      interface (trailer.h) smaller, and

  (2) external API users are unable to peer inside this struct (because
      it is only ever exposed as an opaque pointer).

There are a few disadvantages:

  (A) every time the member of the struct is accessed an extra pointer
      dereference must be done, and

  (B) for users of trailer_info outside trailer.c, this struct can no
      longer be allocated on the stack and may only be allocated on the
      heap (because its definition is hidden away in trailer.c) and
      appropriately deallocated by the user, and

  (C) without good documentation on the API, the opaque struct is
      hostile to programmers by going opposite to the "Show me your
      data structures, and I won't usually need your code; it'll
      be obvious." mantra [2].

(The disadvantages have already been observed in the two preparatory
commits that precede this one.) This commit believes that the benefits
outweigh the disadvantages for designing APIs, as explained below.

Making trailer_info private exposes existing deficiencies in the API.
This is because users of this struct had full access to its internals,
so there wasn't much need to actually design it to be "complete" in the
sense that API users only needed to use what was provided by the API.
For example, the location of the trailer block (start/end offsets
relative to the start of the input text) was accessible by looking at
these struct members directly. Now that the struct is private, we have
to expose new API functions to allow clients to access this
information (see builtin/interpret-trailers.c).

The idea in this commit to hide implementation details behind an "opaque
pointer" is also known as the "pimpl" (pointer to implementation) idiom
in C++ and is a common pattern in that language (where, for example,
abstract classes only have pointers to concrete classes).

However, the original inspiration to use this idiom does not come from
C++, but instead the book "C Interfaces and Implementations: Techniques
for Creating Reusable Software" [1]. This book recommends opaque
pointers as a good design principle for designing C libraries, using the
term "interface" as the functions defined in *.h (header) files and
"implementation" as the corresponding *.c file which define the
interfaces.

The book says this about opaque pointers:

    ... clients can manipulate such pointers freely, but they can’t
    dereference them; that is, they can’t look at the innards of the
    structure pointed to by them. Only the implementation has that
    privilege. Opaque pointers hide representation details and help
    catch errors.

In our case, "struct trailer_info" is now hidden from clients, and the
ways in which this opaque pointer can be used is limited to the richness
of <trailer.h>. In other words, <trailer.h> exclusively controls exactly
how "trailer_info" pointers are to be used.

[1] Hanson, David R. "C Interfaces and Implementations: Techniques for
    Creating Reusable Software". Addison Wesley, 1997. p. 22

[2] Raymond, Eric S. "The Cathedral and the Bazaar: Musings on Linux and
    Open Source by an Accidental Revolutionary". O'Reilly, 1999.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Linus Arver <linus@ucla.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>