Update namespaces to support metadata

[djblue]

Fixes #605

% jank repl
user=> (+ 1 2 3)
6

user=> (ns foo {:a 1})
nil

foo=>  (ns ^{:a 1} foo)
nil

foo=>  (meta *ns*)
{:jank/source {:file "/tmp/jank-repl-aeq2nT", :start {:offset 12, :line 1, :col 13}, :end {:offset 15, :line 1, :col 16}}, :a 1}

[djblue]

I wonder if we should split metadatable into two separate concepts? Some objects in Clojure have metadata, but don't support with-meta, so it feels like we need a concept that says: this objects supports reading metadata, but says nothing about how that metadata is set.

Also, I know this should be in its own file, putting it here temporarily.