Redo syntax groups

* Class level declarative macros (default_scope, helper_method, etc.)
  now all directly use or link to rubyMacro, which by default is
  highlighted as PreProc.

* Highlight attribute declarations (class_attribute, cattr_accessor,
  etc.) the same as attr_accessor.  On the latest vim-ruby this uses
  rubyMacro, but earlier versions use Statement.

* Use rubyEntity and rubyEntities for declarative macros that accept a
  singular (has_one) or plural (has_many) resource name, for possible
  introspection in gf.  Both of these link to rubyMacro.

* Use rubyException for test assertions, which are essentially
  wrappers around raise.

* For everything else, link to Function if it's primarily invoked for
  a return value, or Statement if it's primarily invoked for side
  effects.  I'm particularly interested in feedback in this area, as
  sometimes it can be more confusing than helpful.

* Keyword projections are almost certainly going away, but for now,
  treat them as Function in helpers and views, and rubyMacro everywhere
  else.  This matches the most common use cases.

Author: tpope