[libc++][format][1/6] Reduce binary size.

This removes the `format_args_t` from `<format>` and adjusts the type of
the `format_args` for the `vformat_to` overloads.

The `format_context` uses a `back_insert_iterator<string>` therefore the
new `output_iterator` function uses a `string` as its temporary storage
buffer. This isn't ideal. The next patches in this series will improve
this. These improvements make it easy to also improve `format_to_n` and
`formatted_size`.

This addresses P2216 `6. Binary size`.
P2216 `5. Compile-time checks` are not part of this change.

Implements parts of:
- P2216 std::format improvements

Depends on D103670

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D110494