[0.10] Apply streaming trait directly to shapes

[JordonPhillips]

This updates the streaming trait to apply directly to shapes instead of
members. This was previously restricted to members because it made the
model easier to validate and grok at a glance. However, keeping it on
members introduces potential errors in code generation. A given blob
could have both the streaming trait and the media type trait, for
instance. Both of these traits could result in a new type being created,
but since the streaming trait is applied to a member that type is
implicit. The code generator would have to try to generate a name that
doesn't conflict with any existing names, which is very error prone and
likely would result in unfortunate names. Moving this trait to the
shape directly makes the naming concern an explicit part of modeling,
where it will only have to be handled once.

This also brings the trait in line with the general best practice of
having traits that impact the type generated apply directly to the
shapes they impact.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.