Make the EGraph pattern matcher instantiate to ENodes instead of actual expressions with eclasses as children.

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This issue gets in the way for #82 .
See

Metatheory.jl/src/EGraphs/ematch.jl

Lines 37 to 47 in edfa891

	# FIXME instantiate function object as operation instead of symbol if present!!
	function instantiate(g::EGraph, pat::PatTerm, sub::Sub, rule::AbstractRule)
	eh = exprhead(pat)
	op = operation(pat)
	ar = arity(pat)
	T = gettermtype(g, op, ar)
	println(T)
	children = map(x -> instantiate(g, x, sub, rule), arguments(pat))
	similarterm(T, op, children; exprhead=eh)
	end

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Done in 2.0