Call %whatis as a real hook function, remove $&whatis, and call %run to run binaries

[jpco]

This is an extension to #90 and fixes #80.

This PR keeps resolution of functions within eval.c, but for anything else, it calls out to the function %whatis with the term it is looking up as well as the function name if applicable. Whatever %whatis returns, es then attempts to evaluate.

%whatis is simply defined as

fn %whatis term name {
	if {~ $#name 0} {
		name = $term
	}
	if {!~ $#(fn-$term) 0} {
		result $(fn-$term)
	} {~ $term /* ./* ../*} {
		result %run $term $name
	} {
		let (searched = <={%pathsearch $term})
		result %run $searched(1) $name $searched(2 ...)
	}
}

%whatis does a redundant function lookup for the sake of users calling whatis or %whatis, but then looks for a binary and, if it finds one, returns a %run invocation with the binary as an argument. As in #90, $&run is the only part of the shell actually responsible for invoking binary child processes.

I think this is all surprisingly tidy. It makes both %whatis and %run work as hook functions like they both have always seemed like they should, it works fine with lexical functions without weirdness like $&keeplexicalbinding, and it's even some 20 fewer lines of code compared to the current behavior.