Make storage order for `bitmap!` structs configurable

Currently, the following code:
```rust
bitmap!(
    struct Bits {
         a: u8,
         b: u8,
    }
)
```
will always store the fields as follows:
| 15-8 | 7-0 |
|--------|--------|
| a | b |

This is by choice, as big-endian is often easier to reason about, but it might be cool to have the choice. 