sync: faster Cond
The new version does not require any memory allocations and is 30-50% faster.
Also detect and painc if Cond is copied after first.

benchmark            old ns/op    new ns/op    delta
BenchmarkCond1             317          195  -38.49%
BenchmarkCond1-2           875          607  -30.63%
BenchmarkCond1-4          1116          548  -50.90%
BenchmarkCond1-8          1013          613  -39.49%
BenchmarkCond1-16          983          450  -54.22%
BenchmarkCond2             559          352  -37.03%
BenchmarkCond2-2          1916         1378  -28.08%
BenchmarkCond2-4          1518         1322  -12.91%
BenchmarkCond2-8          2313         1291  -44.19%
BenchmarkCond2-16         1885         1078  -42.81%
BenchmarkCond4            1070          614  -42.62%
BenchmarkCond4-2          4899         3047  -37.80%
BenchmarkCond4-4          3813         3006  -21.16%
BenchmarkCond4-8          3605         3045  -15.53%
BenchmarkCond4-16         4148         2637  -36.43%
BenchmarkCond8            2086         1264  -39.41%
BenchmarkCond8-2          9961         6736  -32.38%
BenchmarkCond8-4          8135         7689   -5.48%
BenchmarkCond8-8          9623         7517  -21.89%
BenchmarkCond8-16        11661         8093  -30.60%

R=sougou, rsc, bradfitz, r
CC=golang-dev
https://codereview.appspot.com/11573043