Rename byteAt to getByte.

Should we later support random access for other primitives
or random bulk access, I'd like the prefix to stay constant
(getByte, getInt, getLong, getBytes) vs. the suffix (byteAt,
intAt, longAt). Prefixing may work better for autocomplete
in IDEs, particularly since we already use a prefix for our
consuming reads (readByte, readInt, readLong).