feat(fmt)!: Generalize custom formatter to 'RecordFormat'

[epage]

This breaks type inference for basic custom formatters

[coveralls]

Pull Request Test Coverage Report for Build 14844590081

Details

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.4%) to 43.76%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/fmt/mod.rs	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/fmt/mod.rs	4	62.5%

Totals
Change from base Build 14242549930:	-0.4%
Covered Lines:	263
Relevant Lines:	601

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💛 - Coveralls

[djugei]

I have tested a similar change for #363 and i recommend against it.

Too much type info annoyance. I would instead recommend having two separate functions, one for the common case (FormatFn) and one for special needs (RecordFormat)