compress/flate: reduce allocations

compress/flate: reduce allocations

benchmark          old ns/op    new ns/op    delta
BenchmarkGunzip   8628248473   8509271514   -1.38%

benchmark         old allocs   new allocs    delta
BenchmarkGunzip       209831       172185  -17.94%

benchmark          old bytes    new bytes    delta
BenchmarkGunzip    579662000    300007152  -48.24%

Update Issue 6317

[rsc, 2013-12-18 02:42:23]

not lgtm

we went through this with bufio. 
no recycling behind the scenes. use Reset methods.