Resolved namespaced enum errors in gfx_macros

I simplified the type names a bit - thought it might be nicer now that we can use the enum name-spacing (but let me know if you want it changed back).

I'm still getting this issue

```shell

   Compiling device v0.1.0 (file:///Users/Mitch/Programming/Rust/gfx-rs)
../gl_device/lib.rs:29:17: 29:28 error: unresolved import `MapReadable`. There is no `MapReadable` in `???`
../gl_device/lib.rs:29 use {MapAccess, MapReadable, MapWritable, MapRW, ReadableMapping, WritableMapping, RWMapping,
                                       ^~~~~~~~~~~
../gl_device/lib.rs:29:30: 29:41 error: unresolved import `MapWritable`. There is no `MapWritable` in `???`
../gl_device/lib.rs:29 use {MapAccess, MapReadable, MapWritable, MapRW, ReadableMapping, WritableMapping, RWMapping,
                                                    ^~~~~~~~~~~
../gl_device/lib.rs:29:43: 29:48 error: unresolved import `MapRW`. There is no `MapRW` in `???`
../gl_device/lib.rs:29 use {MapAccess, MapReadable, MapWritable, MapRW, ReadableMapping, WritableMapping, RWMapping,

```

Not quite sure what's going on here?