Support defrecord reader tags

Currently, defrecord instances print similar to how they do in clojure

> (pr-str (garden.units/px 5))
#garden.types.CSSUnit{:unit :px, :magnitude 5}

This representation cannot be read by the compiler, nor at runtime by cljs.reader/read-string

> #garden.types.CSSUnit{:unit :px, :magnitude 5}
clojure.lang.ExceptionInfo: garden.types.CSSUnit {:type :reader-exception, :line 1, :column 22, :file "NO_SOURCE_FILE"}
...
> (cljs.reader/read-string "#garden.types.CSSUnit{:unit :px, :magnitude 5}")
#<Error: Could not find tag parser for garden.types.CSSUnit in ("inst" "uuid" "queue" "js")>
...

Analysis

The two requirements - using record literals in cljs source code and supporting runtime reading - can be addressed by using the analyzer to find defrecords and registering them with the two respective reader libraries.

Record literals

Since clojurescript reads and compiles a file at a time, clojure's behavior for literals is hard to exactly mimic. That is, to be able to use the literal in the same file where the record is defined.
A reasonable compromise might be to update the record tag table after each file has been analyzed. Thus the literal form of a record could be used only in requiring files.

EDIT: Record literals can also go into the constant pool

cljs.reader

To play well with minification, the ^:export annotation could be reused on defrecords, to publish the corresponding reader tag to cljs.reader.

Related Tickets

• Reusing the ^:export annotation interacts with http://dev.clojure.org/jira/browse/CLJS-1109
• data_readers.cljc support http://dev.clojure.org/jira/browse/CLJS-1194 and http://dev.clojure.org/jira/browse/CLJS-335