#4114 moves the bodies of Stream::decoded_within and decoded_strict into
decoded_within_typed and decoded_strict_within_typed, which return
FilterError so DecodeBudget can tell a spent limit from corrupt data.
This branch checks /Filter with readable_filters there. Kept #4114's
typed functions and made readable_filters return
FilterError::Unsupported, so a /Filter that is not a name fails both
typed decodes without charging the budget; decoded_first_filter_strict
maps it to CosError as before.
aiongg
0e643d45a0