in-ns needs to create ns if nonexistent

[mfikes]

(in-ns 'foo.core) followed by (def a 3) should work even if the foo.core namespace doesn't yet exist

[arichiardi]

Actually the solution is in planck already :smile: ... https://github.com/mfikes/planck/blob/master/planck-cljs/src/planck/repl.cljs#L114