str macro emits unoptimizable js code

Clojurescript's str macro emits javascript code which is inefficient and which the closure compiler cannot optimize.

Currently it emits code like [cljs.core.str(arg1),cljs.core.str(arg2)].join(''). The problems with this:

1. The emitted function is the arity-dispatch str wrapper instead of the 1-arg implementation of str. The closure compiler cannot eliminate the dispatch.

2. An intermediate array is always created; the closure compiler cannot optimize it out.

3. The closure compiler can evaluate constant string expressions (e.g. 'a'+1 to 'a1'), but cannot in this case because it cannot eliminate the str call.

The attached patch rewrites the str macro to generate js code that looks like this:

(str arg1 "constant" \space true nil 123 456.78)

(""cljs.core.str.cljs$core$IFn$_invoke$arity$1(arg1)"constant "+true+123+456.78)

This has a number of benefits:

1. No short-lived array or Array.join operation.

2. No arity dispatch is invoked. I have also observed that it can (but won't necessarily) inline the function body.

3. The compiler can perform constant evaluation. For example, in advanced mode the compiler will emit the above example as (""w.c(a)"constant true123456.78") where w.c is the munged cljs.core.str.cljs$core$IFn$_invoke$arity$1