* Redefine `seq[Blob]` => `seq[SnapProof]` for `snap/1` protocol
why:
Proof nodes are traded as `Blob` type items rather than Nim objects. So
the RLP transcoder must not extra wrap proofs which are of type
seq[Blob]. Without custom encoding one would produce a
`list(blob(item1), blob(item2) ..)` instead of `list(item1, item2 ..)`.
* Limit leaf extractor by RLP size rather than number of items
why:
To be used serving `snap/1` requests, the result of function
`hexaryRangeLeafsProof()` is limited by the maximal space
needed to serialise the result which will be part of the
`snap/1` repsonse.
* Let the range extractor `hexaryRangeLeafsProof()` return RLP list sizes
why:
When collecting accounts, the size oft the accounts list when encoded
as RLP is continually updated. So the summed up value is available
anyway. For the proof nodes list, there are not many (~ 10) so summing
up is not expensive here.
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