reflect: shorten Value to 2 words

reflect: shorten Value to 2 words

This is an experiment.
Method values are not implemented yet.
They will have to set ptr = a pointer to
a struct containing the method index
and the original Value.
That means Method will have to allocate,
but that's not too big a loss.

The new Value is more compact but a little slower:

benchmark               old MB/s     new MB/s     speedup
BenchmarkGobDecode      52.14        51.21        0.98x
BenchmarkGobEncode      58.12        56.42        0.97x
BenchmarkJSONEncode     70.91        68.12        0.96x
BenchmarkJSONDecode     18.98        18.29        0.96x
BenchmarkTemplate       14.62        15.09        1.03x