sharedIndex use Too Much Memory. Add option to let runtime use a standalone index.

[kentalee]

Is your feature request related to a problem? Please describe.

I have large amount of cue file which should be paesed and decoded to struct when program starting.
I found allocated memory does not decrease after my instance set to nil, and many objects still used by core/runtime.sharedIndex
In my case, I do not need cue to hold config information after program loaded.
I hope memory freed after instance destroyed.

Describe the solution you'd like

Add option to core/runtime.New or core/runtime.Init, to let runtime use a clone of sharedIndex. e.g.

// internal/core/runtime/runtime.go

func (r *Runtime) Init() {
	if r.index != nil {
		return
	}
	// r.index = sharedIndex
	r.index = sharedIndex.Clone()
	r.loaded = map[*build.Instance]interface{}{}
}

// internal/core/runtime/imports.go

func (x *index) Clone() *index {
    var n = newIndex()
    // clone data from x ......
    return n
}

Describe alternatives you've considered

Maybe we have some way for user to clear sharedIndex ?

Additional context

[kentalee]

duplicate to #1424