Make dynamic_format_arg_store reusable

[vsolontsov-ll]

It must be easy to make the dynamic_format_arg_store reusable by adding method clear(). Also adding method reserve(num_args, num_named_args) can improve memory management.

[vitaut]

PRs are welcome =).

[vsolontsov-ll]

Will do. The only trouble - I see no simple way to test reserve() method.

[vitaut]

https://github.com/fmtlib/fmt/blob/7f723fbcb8eccc50f6efe988a97db6c9d7f18b28/test/mock-allocator.h if you feel adventurous.

[vsolontsov-ll]

Right....:-)
Well, as of now dynamic_format_arg_store<> is not parameterized by allocator. I'm not infected by TDD that much. Do you see a reason for adding allocator there besides testing trivial method?

[vitaut]

No, I'm OK not testing whether reserve does anything useful since it's just an optimization and doesn't affect correctness. It's enough to test that it doesn't break things.