Test and Benchmark names are too generic

[gaby]

When using msgp for multiple structures in one big code base all the benchmarks are generated with the same name, while this doesn't cause an issue it makes comparing Previous/Current benchmarks impossible.

For example: BenchmarkAppendMsgitem()

When running with go test ./... -benchmem -run=^$ -bench . we get multiple entries with the same benchmark name even though the code are in different folders. Ideally msgp would add a cli flag that allows to specify a name to be appending after the word Benchmark