Remove default hasher

[gui1117]

This is the first step to achieve #4610

First we remove default hasher, later when all conflict has been upgraded, we can introduce a new default.

You can review by commit here, first commit is the change of decl_storage macro, second commit is the fix of all decl_storage calls in substrate.

Note that if the hasher is missing the message printed is:

error: Default hasher has been removed, use explicit hasher(blake2_256) instead.
   --> frame/system/src/lib.rs:479:37
    |
479 |         EventTopics get(fn event_topics): map T::Hash => Vec<(T::BlockNumber, EventIndex)>;
    |                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Companion PR for polkadot paritytech/polkadot#804