When we freeze a BytesMut, we turn it into a Vec, and then convert that
to a Bytes. Currently, this happen using Vec::into_boxed_slice, which
reallocates to a slice of the same length as the Vev if the length and
the capacity are not equal. This can pose a performance problem if the
Vec is large or if this happens many times in a loop.
Instead, let's compare the length and capacity, and if they're the same,
continue to handle this using into_boxed_slice. Otherwise, since we
have a type of vtable which can handle a separate capacity, the shared
vtable, let's turn our Vec into that kind of Bytes. While this does not
avoid allocation altogether, it performs a fixed size allocation and
avoids any need to memcpy.
