`formatter::parse` and `formatter::format` only have to accept
`basic_format_parse_context` and `basic_format_context` respectively,
which are only guaranteed to be provided via `<format>`. This allows us
to only declare the functions for `formatter<bool>` in `<vector>` and
define them if `<format>` is included. This reduces the amount of time
it takes to parse `<vector>` by ~15% on my system.
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