[Module] Remove DataDescription's writable field.

It was redundant, as data object declarations also have a writable
field, so just use that, avoiding the need for users to declare the
same thing twice.

Fixes #456.

lib/module/src/lib.rs

@@ -33,7 +33,7 @@ mod data_context;
 mod module;
 
 pub use backend::Backend;
-pub use data_context::{DataContext, DataDescription, Init, Writability};
+pub use data_context::{DataContext, DataDescription, Init};
 pub use module::{
     DataId, FuncId, FuncOrDataId, Linkage, Module, ModuleError, ModuleNamespace, ModuleResult,
 };