Remove support for implicit self sends and convert everything to '@'.

This fixes the nasty case where "foo(bar)" is context-sensitive, and
generally simplifies the compiler a lot since there is a clear
distinction between lexical and dynamic scope.

Also:

- Remove the special handling of capitalized names since all names are
  lexical now.
- Allow methods to close over local variables in enclosing functions.
- Allow implicit definition of all lexical names.

test/language/module_variable/use_in_function.wren

@@ -0,0 +1,5 @@
+var variable = "module"
+
+Fn.new {
+  System.print(variable) // expect: module
+}.call()