Assigning a value to Function Name in Gno should be prohibited

[omarsy]

Description

The following code snippet compiles and runs successfully in Gno, which is incorrect behavior:

package main

func v() {}
func main() {
	v = func() {}
	println(v)
}

Expected Behavior

This code should produce an error indicating that you cannot assign to the function name v, such as:

cannot assign to v (neither addressable nor a map index expression)