Eliminate str call on literal strings in str macro

The str macro unconditionally calls str on each of its arguments. This can be skipped for literal strings, saving on the unnecessary extra runtime call.

For example: (str "x" x "y") would be compiled down to JavaScript with fewer calls ["x",u.c(a),"y"].join("")
and silly things like (str "a" "b") compile all the way down to "ab".