Add a NoOptionalPointers config option for gocode types gen

[sdboyer]

Currently, optional fields in a schema result in Go code with pointer types, as that's the only means Go has for actually expressing field optionality.

While technically correct, this is very non-ergonomic Go. While it pains me to do it as it introduces an ambiguity (see grafana/grok#1), we should at least add an option to gocode.TypeConfigOpenAPI, say NoOptionalPointers, which causes optional fields not to be generated with pointers.

Additionally, we should just never render slice and map types with pointers. It's redundant.

I wrote this little thing a while ago to fix that, though never committed it:

// MapSliceDepointerizer returns an AST manipulator that removes redundant
// pointer indirection from all map and slice types.
func MapSliceDepointerizer() astutil.ApplyFunc{
       return func(c *astutil.Cursor) bool {
               switch x := c.Node().(type) {
               case *ast.Field:
                       if s, is := x.Type.(*ast.StarExpr); is {
                               switch deref := depoint(s).(type) {
                               case *ast.ArrayType, *ast.MapType:
                                       x.Type = deref
                               }
                       }
               }
               return true
       }
}

It will need to be shifted to dst, but i suspect it's a good basis for handling both of these problems at once.