Refactors all `format` functionality with knowledge of character encodings into a new class `_Fmt_codec`. `_Fmt_codec` internally caches the `_Cvtvec` structure that other functions previously passed around, and makes use of a new "pure C ABI function" `__std_get_cvt` to retrieve character conversion info for the active codepage.

The ABI function reuses the `__std_code_page` and `__std_win_error` types from `<xfilesystem_abi.h>`. These could be extracted into a more general `__msvc_win_abi.hpp` header, but I think `<xfilesystem_abi.h>` is lightweight enough to just include directly for now.

Drive-by:
* BUG: `_Parse_precision` was not skipping the trailing `}` in a dynamic precision.
* Improvement: `test_parse_helper` now verifies that the parse consumes the entire input when passed an expected length of `npos`.

... fix 2 bugs exposed by that coverage, and address review comments. Drive-by: Remove the `_FORMAT_CODEPAGE` override from the `formatting_utf8` test which is unnecessary since we compile it with `/utf-8`.