1. SudoG always contains a pointer to the element
(thus no variable size, and less copying).
2. chansend/chanrecv allocate SudoG on the stack.
3. Copying of elements and gorotuine notifications
are moved out of critical sections.

benchmark                        old ns/op    new ns/op    delta
BenchmarkSelectUncontended          515.00       514.00   -0.19%
BenchmarkSelectUncontended-2        291.00       281.00   -3.44%
BenchmarkSelectUncontended-4        213.00       189.00  -11.27%
BenchmarkSelectUncontended-8         78.30        79.00   +0.89%
BenchmarkSelectContended            518.00       514.00   -0.77%
BenchmarkSelectContended-2          655.00       631.00   -3.66%
BenchmarkSelectContended-4         1026.00      1051.00   +2.44%
BenchmarkSelectContended-8         2026.00      2128.00   +5.03%
BenchmarkSelectNonblock             175.00       173.00   -1.14%
BenchmarkSelectNonblock-2            85.10        87.70   +3.06%
BenchmarkSelectNonblock-4            60.10        43.30  -27.95%
BenchmarkSelectNonblock-8            37.60        25.50  -32.18%
BenchmarkChanUncontended            109.00       114.00   +4.59%
BenchmarkChanUncontended-2           54.60        57.20   +4.76%
BenchmarkChanUncontended-4           27.40        28.70   +4.74%
BenchmarkChanUncontended-8           14.60        15.10   +3.42%
BenchmarkChanContended              108.00       114.00   +5.56%
BenchmarkChanContended-2            621.00       617.00   -0.64%
BenchmarkChanContended-4            759.00       677.00  -10.80%
BenchmarkChanContended-8           1635.00      1517.00   -7.22%
BenchmarkChanSync                   299.00       256.00  -14.38%
BenchmarkChanSync-2                5055.00      4624.00   -8.53%
BenchmarkChanSync-4                4998.00      4680.00   -6.36%
BenchmarkChanSync-8                5019.00      4760.00   -5.16%
BenchmarkChanProdCons0              316.00       274.00  -13.29%
BenchmarkChanProdCons0-2           1280.00       617.00  -51.80%
BenchmarkChanProdCons0-4           2433.00      1332.00  -45.25%
BenchmarkChanProdCons0-8           3651.00      1934.00  -47.03%
BenchmarkChanProdCons10             153.00       152.00   -0.65%
BenchmarkChanProdCons10-2           626.00       581.00   -7.19%
BenchmarkChanProdCons10-4          1440.00      1323.00   -8.12%
BenchmarkChanProdCons10-8          2036.00      2017.00   -0.93%

R=rsc, ken
CC=golang-dev
https://golang.org/cl/4790042