Implement namespaces

[borkdude]

• We need to keep track of what is the current namespace.
• Defining vars will be stored and resolved under the :namespaces key in a mutable atom, since sci can now mutate namespaces
• The default current namespace will be user like clojure.core.
• The babashka socket REPL must reflect the current ns
• Keywords must be autoresolved using the known namespaces, e.g. ::str/foo
• aliases must be stored per namespace, not globally!
• We need to implement an ns form. We already support require and import, these need to be ported to it.
  • require
  • import
• fix JS interop:

$ plk -e "(require '[sci.core :as sci])" -e '(sci/eval-string "(ns foo) (js/Error.)")'
Execution error (ExceptionInfo) at (<cljs repl>:1).
Unable to resolve classname: Error [at line 1, column 11]