validate: refactor validate module to use spec-defined algorithm

Fixes #143.

validate/operand.go

@@ -8,6 +8,20 @@ import (
 	"github.com/go-interpreter/wagon/wasm"
 )
 
+const (
+	noReturn    = wasm.ValueType(wasm.BlockTypeEmpty)
+	unknownType = wasm.ValueType(0)
+)
+
 type operand struct {
 	Type wasm.ValueType
 }
+
+// Equal returns true if the operand and given type are equivalent
+// for typechecking purposes.
+func (p operand) Equal(t wasm.ValueType) bool {
+	if p.Type == unknownType || t == unknownType {
+		return true
+	}
+	return p.Type == t
+}