compiler: move SHA256 from builtins to syscalls

Now it can be processed as a normal syscall.

pkg/compiler/analysis.go

@@ -16,7 +16,7 @@ var (
 	goBuiltins = []string{"len", "append", "panic"}
 	// Custom builtin utility functions.
 	customBuiltins = []string{
-		"SHA256", "AppCall",
+		"AppCall",
 		"FromAddress", "Equals",
 		"ToBool", "ToByteArray", "ToInteger",
 	}

pkg/compiler/codegen.go

@@ -1220,8 +1220,6 @@ func (c *codegen) convertBuiltin(expr *ast.CallExpr) {
 			typ = stackitem.BooleanT
 		}
 		c.emitConvert(typ)
-	case "SHA256":
-		emit.Syscall(c.prog.BinWriter, "Neo.Crypto.SHA256")
 	case "AppCall":
 		c.emitReverse(len(expr.Args))
 		buf := c.getByteArray(expr.Args[0])

pkg/compiler/syscall.go

@@ -36,6 +36,7 @@ var syscalls = map[string]map[string]Syscall{
 		"ECDsaSecp256r1Verify":        {"Neo.Crypto.VerifyWithECDsaSecp256r1", false},
 		"ECDSASecp256r1CheckMultisig": {"Neo.Crypto.CheckMultisigWithECDsaSecp256r1", false},
 		"RIPEMD160":                   {"Neo.Crypto.RIPEMD160", false},
+		"SHA256":                      {"Neo.Crypto.SHA256", false},
 	},
 	"enumerator": {
 		"Concat": {"System.Enumerator.Concat", false},