[READY] Optimize completion item extra data

[bstaletic]

The ycmd protocol says that the client side cares only about extra_data[ 'resolve' ], so we need not serialize the LSP item as well.

Unfortunately, the standard library json has no useful mechanism for skipping certain dict keys. Instead, we implement a custom type that behaves like a dict, but stores the LSP item on the side.

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This change is