Let `defn.sequence_macro` cooperate with an “enclosing” form

A defn.sequence_macro can be used to hook into the partial expansion for block and alter how the following forms should be evaluated, while #%body can be redefined to alter how the whole block is interpreted at all. Currently, we have Closeable.let as a defn.sequence_macro, but it just assumes that it and the following forms should be interpreted as in the default #%body. Is there a good way to let defn.sequence_macro cooperate with user-defined #%body bindings? (This change may need a more refined definition macro protocol, for example, to allow proper partial expansion.)

Start by examining the defn.sequence_macro protocol and how Closeable.let participates in partial expansion. Then compare that behavior with user-defined #%body bindings and the handling of block-following forms. Done means defining a clear protocol that lets sequence macros cooperate correctly with enclosing forms.