Add a derive macro for CustomType

[MavethGH]

The macro currently generates getters and setters for all fields in the struct, and has helper attributes for overriding generated getter/setter functions. There is also a readonly helper attribute that prevents a setter from being registered.

TODOs:

• Docs docs docs
• More tests
• Register a pretty-print name for types
• Attribute for calling a user-provided function with the TypeBuilder passed in
  • What would it be called?
  • Do we call it before or after registering everything else?
• Attribute for skipping a field entirely
• Support for tuple structs
  • What would the fields be named? field0, field1, etc perhaps?
• Support for enums as described in the book

[schungx]

Looking good!

Let me know when to merge.

[schungx]

So below is list of all the attributes so far...

• All fields (pub or private) will be exported
  • perhaps later a skip attribute
• #[readonly] => only getter, no setter
• #[get] => generated getter
  • #[get(func_name)] => use func_name as getter function
• #[set] => generated setter
  • #[set(func_name)] => use func_name as setter function
• omitted => generated getter and setter

Did I miss anything?

[schungx]

Unfortunately we cannot make #[get(func_name)] use a standard getter signature with &self instead of &mut self which is quite unusual for normal structs...

[schungx]

I'll open another tracking issue...