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We support indefinite length bytestring in the Read module. In order to do it we add two extra variants for bigint deseralization, both puts us to the slow-path to start consumption of the chunks. This serialization is considered non-canonical thus canonical read rejects is. While specification is silent about it, the cbor.me implementation always converts bigit into a fixed size byte blob. Fixes well-typed#263.
The previous patch, missed a case in which incremental decoding is fed with `0xc2` (or `0xc3`) indicating that we are decoding big integer, but then the `0x5f` (`TypeBytesIndef`) is in the next chunk. The original implementation ended up in `adjustContBigUIntNeedHeader` or `adjustContBigNIntNeedHeader` which issues `ConsumeBytes`, which fails on `TypeBytesIndef`. For that reason both `adjustContBig[UN]IntNeedHeader` need to first peek and verify weather we are at `TypeBytesIndef` (`0x5f`) or passed it. This fix is not required for `adjustContCanonicalBig[UN]IntNeedHeader` since canonical encoding of `Integer`s must not use indefinite encoding, and it is rightfully rejected by `tryConsumeBytes`.
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This PR subsumes #371 - it fixes a subtle bug in incremental decoding of big integers, also provides a test which shows the bug and verifies it is fixed.