Have the decl_storage! macro import rstd::prelude::*

[shawntabrizi]

When trying to use the decl_storage macro and build to wasm, the compiler complains that the Vec type has not been declared.

For example, this minimum runtime:

use srml_support::{StorageValue, dispatch::Result};
pub trait Trait: system::Trait {}

decl_module! {
  pub struct Module<T: Trait> for enum Call where origin: T::Origin {
    fn set_value(_origin, value: u32) -> Result {
      <Value<T>>::put(value);
      Ok(())
    }
  }
}

decl_storage! {
  trait Store for Module<T: Trait> as RuntimeExampleStorage {
    Value: u32;
  }
}

will give the following error:

error[E0412]: cannot find type `Vec` in this scope
  --> src/runtime_example.rs:13:1
   |
13 | / decl_storage! {
14 | |   trait Store for Module<T: Trait> as RuntimeExampleStorage {
15 | |     Value: u32;
16 | |   }
17 | | }
   | |_^ not found in this scope
help: possible candidates are found in other modules, you can import them into scope
   |
1  | use rstd::alloc::prelude::Vec;
   |
1  | use rstd::prelude::Vec;
   |
1  | use rstd::vec::Vec;
   |
1  | use srml_support::dispatch::Vec;
   |
and 1 other candidates

error: aborting due to previous error

For more information about this error, try `rustc --explain E0412`.
error: Could not compile `runtime-example-runtime`.

To solve this issue, you simply need to add: use rstd::prelude::*;

We should have the decl_storage macro automatically do this if it requires this import.