Don't consume readers with FixedLengthReader

Don't consume readers with FixedLengthReader

We can't always assume that we're done reading after using a
FixedLengthReader. In some cases, we may need to read a set of
length-prefixed objects one at a time, and we'd like to do so without
incurring an additional allocation by reading a number of bytes first to
then deserialize them.