Re-export amount from primitives

We are trying to make it so that what ever crate a user uses they see
the same module/type tree (if the module or type exists).

E.g., one can do either of these.

If they use `bitcoin`:

```
use bitcoin::{
    amount, block, fee_rate, weight, merkle_tree, opcodes,
    pow, script, sequence, transaction, witness,
};
```

Or if they use `primitives`:

```
use bitcoin_primitives::{amount, block, fee_rate, weight};
```

The above imports were tested and `amount` was found to be missing.

primitives/src/lib.rs

@@ -44,7 +44,7 @@ pub mod transaction;
 pub mod witness;
 
 #[doc(inline)]
-pub use units::amount::{Amount, SignedAmount};
+pub use units::amount::{self, Amount, SignedAmount};
 #[cfg(feature = "alloc")]
 #[doc(inline)]
 pub use units::{