compiler: emit Neo.Crypto.ECDsaVerify syscall instead of CHECKSIG

Also change the name of `VerifySignature` interop, to match
syscall's name. It also accepts arguments in different order.

pkg/compiler/syscall.go

@@ -1,6 +1,9 @@
 package compiler
 
 var syscalls = map[string]map[string]string{
+	"crypto": {
+		"ECDsaVerify": "Neo.Crypto.ECDsaVerify",
+	},
 	"storage": {
 		"GetContext": "Neo.Storage.GetContext",
 		"Put":        "Neo.Storage.Put",