Research and implement new `layout` metadata format

[cmichi]

Blocker for use-ink/cargo-contract#101.

I'm just copy/pasting from the discussion there:

The current metadata format cannot represent some more or less common scenarios and really needs some restructuring. When I tried some weeks ago I immediately hit a wall for recursive data structures.

We first have to define semantics, definitions and structures for a new and better layout metadata section before we should tackle this issue. We have to think about cases like:

• recursively defined data structures such as enum Tree { Leaf(i32), Node(Box<Tree>, Box<Tree>) } or in case of ink_storage: ink_storage::Vec<ink_storage::Vec<i32>>, packed elements where some elements actually have some further pointers Pack<(i32, ink_storage::Box<i32>) and more.

Maybe we have to restrict the final metadata to not support certain use cases because their metadata format would become too complex otherwise.

[HCastano]

In #1331 we updated the how the storage was defined in the metadata. I believe it should be parsable now, allowing to work on use-ink/cargo-contract#101.

cc @xermicus