chore(rlp): add comment on 0x80 decoding

[Rjected]

The previous method correctly decodes 0x80, but this makes it an explicit case and adds a comment about what happens when 0x80 is decoded as an integer.

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[mattsse]

by no means an rlp expert, so curios what @rakita thinks

[shemnon]

It's more explicit than having to dive into from_big_endian and from_be_bytes to see how they deal with zero-length arrays.

[rakita]

rlp is generally simple to get (even with complex rules that it has), This seems like a good writing: https://ethereum.org/en/developers/docs/data-structures-and-encoding/rlp/

This is just shifting some code to be more as shemnom said explicit, if you want move rest of checks from https://github.com/vorot93/fastrlp/blob/468fdf24608737d979e9b41276c3b91a47981c55/src/decode.rs#L168-L185 to appropriate place

[Rjected]

makes sense, we can check which ones should be moved in a follow up