Also adds fwide byte/wide orientation checking to verify streams are
able to receive the character type in question.

On Windows, the fwide calls are no-ops that pass through the second
arg and optimize out the if statement entirely.

This allows fmt to be compiled with `-fno-exceptions`.

Refactor `format_specs` and related APIs to support variable-width fill
(#1109), improve naming consistency, remove legacy setters (#940), and
optimize layout.