interp: improve handling of aliased types

Avoid to test directly for a type category, as it may give wrong
results for aliased types, where the interesting category remains
masked.  Instead, use some property helpers, such as isFuncSrc,
isPtrSrc and isInterfaceSrc to check if a type is of source function,
source pointer or source interface respectively (versus runtime
defined function, pointer or interface).

Fixes #1068.

_test/issue-1068.go

@@ -0,0 +1,19 @@
+package main
+
+type I interface {
+	Hello()
+}
+
+type T struct{}
+
+func (t T) Hello() { println("hello") }
+
+type I2 I
+
+func main() {
+	var i I2 = T{}
+	i.Hello()
+}
+
+// Output:
+// hello

interp/cfg.go

@@ -576,7 +576,7 @@ func (interp *Interpreter) cfg(root *node, importPath string) ([]*node, error) {
 					dest.gen = nop
 				case isFuncField(dest):
 					// Setting a struct field of function type requires an extra step. Do not optimize.
-				case isCall(src) && dest.typ.cat != interfaceT && !isRecursiveField(dest) && n.kind != defineStmt:
+				case isCall(src) && !isInterfaceSrc(dest.typ) && !isRecursiveField(dest) && n.kind != defineStmt:
 					// Call action may perform the assignment directly.
 					n.gen = nop
 					src.level = level