various ideas for things to detect in a Perl 6 program

Things to check for:

* Referring to variables that aren't in scope.
* Inheriting from classes or roles that weren't declared.
* Presupposing 'self' in a sub.
* Never using 'self' (explicitly or implicitly) in a method or submethod.
* Calling an only sub/method with the wrong number/type of argument.
* Make sure that 'handles' values refer only to methods that are actually
  defined by the given type.
* Check if assumtions are made about parameters which are not checked
  beforehand, such as 'sub foo(@arrays) { for @arrays -> @array { ... } }'
* Subs which are never called.
* 'Inconsistent' use of $^foo and $foo in a scope.
* Reading from a variable before it has been assigned to.
* Type mismatch in assignment.
* Boolean values which are never true, or never false.
* Assigning to a variable and then never using it.
* Just falling out of a sub or method without returning anything. (Need to
  check if the last value appears to be a read of some sort, or the single
  value of the sub or method.)
* use-ing a module and then never referring to it or anything in it.
* Passing a constant which doesn't conform to the where clause in an only
  sub
* Passing 'map' a Hash instead of a Code.
* Having two multi variants which will lead to ties. For example (Int $a?)
  and (Str $a?).