Update benchmark for gogen-avro v10

[actgardner]

Hi! I'm the maintainer of gogen-avro. Thanks for putting together this comparison, it motivated me to make some performance improvements I've been thinking of for a while. The new v10 release includes these changes.

Running your benchmark locally on v9:

cpu: Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
BenchmarkGoGenAvroDecode-12    	  493148	      2171 ns/op	     728 B/op	      45 allocs/op
BenchmarkGoGenAvroEncode-12    	 1618570	       739.8 ns/op	     256 B/op	       3 allocs/op
PASS
ok  	github.com/actgardner/gogen-avro/v9/test/benchmark	3.056s

Running your benchmark locally on v10:

cpu: Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
BenchmarkGoGenAvroDecode-12    	  768646	      1401 ns/op	     320 B/op	      11 allocs/op
BenchmarkGoGenAvroEncode-12    	 1633623	       756.5 ns/op	     256 B/op	       3 allocs/op
PASS
ok  	github.com/actgardner/gogen-avro/v10/test/benchmark	3.085s