Add Sum512() and Sum256() functions.

Go 1.2 standard library introduces such functions for all their hashes,
so we follow the example:

	Sum512(data) returns a 64-byte hash of the data,
	Sum256(data) returns a 32-byte hash of the data.

This commit also restructures internal initialization and finalization
code, removing unnecessary config validation and error checking from
New256() and New512() functions.

Changed BenchmarkHash* benchmarks to use these functions.