Moved files from ast to atoms and changed references in the sources.mlb file.
References refactored
smlnj compilation files updated accordingly
Cleanup `sources.mlb` and `sources.cm` files.

In particular, drop references to `../ast/sources.mlb` and
`../ast/sources.cm`, since the necessary structures are now provided
by `../atoms/sources.mlb` and `../atoms/sources.cm`.
Move `signature WRAPPED` to `control` directory.
Drop unused `structure Atoms.Vars : SET`.
Complete `sharing` constraints for `signature ATOMS`.
Reorganize `atoms/sources.{cm,mlb}`.
Merge pull request #92 from MatthewFluet/RestructuringAtoms

Move pervasive constructs from `ast` to `atoms`.

Most program constructs (e.g., `WordX` for word constants, `Label` for basic block labels) that are used in multiple ILs are defined in the `atoms` directory. However, some program constructs (e.g., `WordSize` for word constants, `Field` for record fields) that are used in multiple ILs are defined in the `ast` directory.

This commit moves all of the pervasive program constructs that are defined in `ast` to `atoms`, so that `ast/sources.mlb` depends on `atoms/sources.mlb` and not the other way around.