hash reorg.

hash reorg.
* new package hash defining interfaces Hash and Hash32.
* adler32 and crc32 return Hash32 instead of specific types.
* adler32 and crc32 provide non-allocating methods for single slices.
* sha1 and md5 move to crypto, return Hash.
* sum.go, a simple test program, moves to /usr/rsc.
* refresh Make.deps

R=r
DELTA=1908  (935 added, 923 deleted, 50 changed)
OCL=29095
CL=29135