Move definition of Feature next to definitions of WorldImpact and Use

pkg/compiler/lib/src/code_organization.md

Index: pkg/compiler/lib/src/code_organization.md
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+[comment]: WIP - code is being refactored...
+
+##General code organization of this pacakge
+
+  lib/src/
+    |- ...
+    |- universe/  - how we represent the closed-world semantics of a program.
+
+[comment]: TODO fill in the rest
+
+
+##Details
+
+### Universe
+[comment]: TODO rename universe => world
+[comment]: TODO consider merging feature.dart and use.dart
+
+How we represent the closed-world semantics of a program.
+
+universe/
+ |- feature.dart - Features that may be used in the program. Using a feature
+ |                 pulls in special code that the compiler needs to support it.
+ |- use.dart     - Describes a use of an element (a method, a class) and how it
+ |                 is used by the program.
+
+