Change visitor code to generate code with explicit self

[jesse99]

Tried to add #[warn(deprecated_self)]; to https://github.com/jdm/rust-socket and got a bunch of warnings like:

<intrinsic>:32:21: 32:25 warning: this method form is deprecated; use an explicit `self` parameter or mark the method as static
<intrinsic>:32         fn visit_bot() -> bool;
                                    ^~~~
<intrinsic>:33:21: 33:25 warning: this method form is deprecated; use an explicit `self` parameter or mark the method as static
<intrinsic>:33         fn visit_nil() -> bool;
                                    ^~~~
<intrinsic>:34:22: 34:26 warning: this method form is deprecated; use an explicit `self` parameter or mark the method as static
<intrinsic>:34         fn visit_bool() -> bool;
                                     ^~~~

This is with 0.5

[catamorphism]

Changed the title to better reflect what the issue is.

[pcwalton]

Fixed.