Replacement operators for firm closures

[egnha]

These would accompany the component extractors firm_core(), firm_checks(), firm_args(), firm_error(). They should apply only to firm closures.

Example: Replace the (input validation) checks of one function by those of another

f <- firmly(function(x, y) NULL, ~is.numeric) 
g <- firmly(function(x, y) NULL, ~is.character)

firm_checks(f) <- firm_checks(g)

[egnha]

Package frozen at 1.0.0.