This patch refactors `length_delimited` to be implemented as a `Codec` and
use the default `Framed` wrapper types.
The original implementation did not do this in order to support vectored writes in the
write half. However, this implementation would be more efficient with small frames anyway.
If vectored writes are to be explored in the future, then it should be done holistically.
Signed-off-by: Eliza Weisman <[email protected]>
