Macro defs are now written like `macro def name(args) : ReturnType` instead of `def name(args) : ReturnType`, which was a bit confusing. Partially related to #160

Replaced `@name` with `@class_name` inside macros. And: instance vars inside macros don't have the `@` in their name anymore.

Reference#to_s is no longer a primitive: it's done with macros. Also: some fixes related to macros and hierarchy metaclasses. Moved Class logic to "class.cr".

Explicit control of AST nodes inside macros. Must use "id" to get an id, and "stringify" to make a string literal with the node's string representation. No more implicit "id" call.

Faster and more accurate way to copy def macros from a base class. Started adding exec_recursive in some places.

Added @instance_vars and @name inside macros. Removed hardcoded Struct#to_s and Struct#hash and implemented them with macros.

Added def macros: def foo : Int32; <macro body>; end. They are expanded after the type inference and the type must match.