Ugly workaround for #2566 issue

[bozaro]

Based on PR (only the last commit is important):

• internal/core/eval: add testcase for https://github.com/cue-lang/cue/issues/2566 issue #2588

This change breaks backward compatibility, but I couldn't think of anything better.

I try explain this workaround by example cue/testdata/eval/let_funcs.txtar:

import "strings"
#Foo: {
	#a: string
	#b: string
	foo: strings.HasPrefix("\(#a)", #b)
}
foo: {
	let env_context = #Foo & {
		#a: "hello"
		#b: "world"
	}
	e1: env_context
	e2: env_context
	e3: env_context
	e4: env_context
	e5: env_context
	e6: env_context
	e7: env_context
	e8: env_context
}

In original code:

• let env_context call strings.HasPrefix at the first time and create finazlied *adt.Vertex (vertex_1). This *adt.Vertex also contains *adt.CallExpr somewhere in conjuncts;
• e1: env_context create new *adt.Vertex (vertex_2). On creating vertex_2 executed n.addVertexConjuncts(v, arc, false) with vertex_1 as arc;
• addVertexConjuncts internally add all conjuncts from vertex_1 to vertex_2;
• vertex_2 finalization reevaluates *adt.CallExpr from added conjuncts.

After this workaround vertex_1 became data and lost conjuncts with *adt.CallExpr node.