Added Read/WriteExtensionRaw

[mhr3]

Our codebase is using Extensions quite extensively, and we've noticed that reading and writing an Extension with the current msgp API always causes an allocation, which is usually not the case when using any other msgp Read*/Write* methods. Therefore we've added Reader.ReadExtensionRaw() and Writer.WriteExtensionRaw() in this PR which allows zero-alloc code paths for extensions.

goos: darwin
goarch: arm64
pkg: github.com/tinylib/msgp/msgp
BenchmarkExtensionReadWrite/interface-8         	   13196	     89295 ns/op	    8377 B/op	       1 allocs/op
BenchmarkExtensionReadWrite/raw-8               	   13910	     88740 ns/op	    8299 B/op	       0 allocs/op
PASS
ok  	github.com/tinylib/msgp/msgp	4.329s