feat: recursive metadata structure

[tjjfvi]

Resolves #284

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Description

This changes the structure of the metadata to be recursive; where before compound types would reference other types via indices, they now reference the type object itself directly. This uses the new context api in scale v0.6.0 to decode directly into this structure, without any need for post-processing.

This eliminates almost all cases where one would need to index into the metadata.tys array, cleaning up the code significantly.

The only part that became more complicated was Metadata.test.ts. Though Deno's snapshot serializer does support circular references, it only uses references to parent objects in the circularity. This meant that it produced a – though finite – pathologically large string, as each type was written in expanded form potentially hundreds of times. To resolve this, Metadata.test.ts now has custom logic to serialize the metadata in a less pathological way. It's currently not exported anywhere, but it could be in the future – though I don't think this would necessarily be of value anywhere else.

If one decodes with the $ty codec standalone (i.e. not by way of $tys or $metadata), it will not have a way to get the types references by the indices in the buffer, so it will instead decode them as dummy objects with an appropriate id field. Though this is not type-safe, I think this is the best middleground.