This CL is only cut-and-paste, moving code around.
Moving it in a separate CL should simplify the diffs in later CLs.

There are three patterns here.

1. A function like
        func (v Value) M() (...) {
                return v.panicIfNot(K).(*kValue).M()
        }
becomes
        func (v Value) M() (...) {
                vv := v.panicIfNot(K).(*kValue)

                // body of (*kValue).M, s/v./vv./g
        }

2. A function like
        func (v Value) M() (...) {
                return v.panicIfNots(kList).(mer).M()
        }
becomes
        func (v Value) M() (...) {
                switch vv := v.panicIfNots(kList).(type) {
                case *k1Value:
                        // body of (*k1Value).M, s/v./vv./g
                case *k2Value:
                        // body of (*k2Value).M, s/v./vv./g
                ...
                }
                panic("not reached")
        }

3. The rewrite of Value.Set follows 2, but each case
is built from the bodies of (*kValue).SetValue and (*kValue).Set.
        func (v *kValue) SetValue(x Value) {
                v.Set(x.panicIfNot(K).(*kValue)
        }
        func (v *kValue) Set(x *kValue) {
                ... body
        }
becomes, in the switch from 2,
                case *kValue:
                        xx := x.panicIfNot(K).(*kValue)
                        ... body, s/v./vv./g; s/x./xx./g

R=r
CC=golang-dev
https://golang.org/cl/4398044