portable: Retain the provided type IDs

[lexnv]

This PR implements the retain method on the PortableRegistry.

The retain method allows tools (such as subxt or capi) to interpret the
RuntimeMetadata for only a subset of pallets.

The method retains only the portable types needed to express the provided ids.
A given type ID can be defined by nesting type IDs, such as the case
of a [TypeDef::Composite] and others. To retain a valid [PortableRegistry]
all the types needed to express an ID are included. Therefore, the number of
elements defined by the result equals or exceeds the number of
provided IDs.

This is implemented in two steps:

• recursively visit and collect all type IDs needed to express the given list of IDs
  • visiting happens in DFS manner
  • while visiting, type IDs are normalized (assigned a new incremental ID starting from zero)
• modify the nested type IDs to point to the new location of the type in the registry

Testing Done

• unit tests
• tested in subxt with PoC branch: https://github.com/paritytech/subxt/tree/lexnv/pallet_metadata_poc

The following represents the metadata size for only one pallet, based on the PoC branch.
Further work is required to minimize the runtime type present in the Metadata to
reduce the size even more than the PoC.

232K Vesting.scale
235K VoterList.scale
232K Whitelist.scale
451K polkadot_metadata.scale

A more realistic number lies between the two type ID dumps from paritytech/subxt#629 (comment).

This is part of: paritytech/subxt#629

// CC: @paritytech/tools-team

[lexnv]

While at it, I've also tried to use the PortableRegistry to build out types without manually updating the nested type ids.

Registry from retain_recursive_ids test

0: u32
1: u64
2: Sequence { ty id 0}
3: MyStruct { primitive: 0, vec_of_u32: 2}
4: MyStructSecond { vec_of_u32: 2, second: 3}

Using the PortableRegistryBuilder without updating nested type IDs

0: MyStructSecond { vec_of_u32: 2, second: 3}
1: Sequence { ty id 0 }
2: u32
3: MyStruct { primitive: 0, vec_of_u32: id 2}

I'm afraid that's not right because MyStruct.primitive should be u32 with ID 2, not ID 0.

Using the current PR

0: MyStructSecond {vec_of_u32: 1, second: 3}
1: Sequence {ty id 2}
2: u32
3: MyStruct { primitive: 2, vec_of_u32: 1 }

[jsdw]

Looks great :)