Forward read_exact() as well as read(). (#207)
Forward read_exact() as well as read(). (#207)

If we don't do this we end up using the generic read_exact method
which is not necessarily optimal. This is especially when
using a specialized Read implementation to go fast.
See https://github.com/TyOverby/bincode/issues/206
servo/bincode,TyOverby/bincode