Implement 'Map namespace syntax'

[jfacorro]

Introduced in Clojure 1.9 (see here).

The implementation for this could add a reader dispatch for keywords (Erlang atoms) here, where instead of directly calling read_tagged/1, it could figure out if the next form is a keyword, symbol or something else (which will generate an error) and continue from there.

[jfacorro]

Update doc/differences-with-clojure.md when/if implemented.