Nested map destructuring in `mx/defn` wraps extra collection

[frenchy64]

The following blows up when instrumented:

(mx/defn foo [{{inner :inner} :outer}]
  inner)
(foo {:outer {:inner true}})

The problem is that the inner map schema is wrapped in an :altn, which turns the outer schema into a [:map [:outer [:altn ...]]]. This turns the inner map into a sequence spec.

There's a potential fix here: https://github.com/metosin/malli/pull/1019

But I'm unsure how this might interact with other features like auto-flattening: https://github.com/metosin/malli/issues/1005