* Simplified compiler by adding function jsfier.purecall() which
   translates the code into either a $PY.call() proxy, or a direct
   prefix-self call if the self value can be guaranteed to be
   side-effect free (currently, if it is of type IName)
 * Cleaned up CLI code, and allow the CLI program to run files directly
 + Added comprehensive slicing test
 + Make casetest output the output when js and py matches
 + Added a new generate option: import-each. This generates a load()
   line for each piece of the stdlib. This makes it much easier to do
   debugging, since line numbers and filenames in backtraces will be
   correct.
 + Enable casetest to use $JS to explicitly specify javascript engine
 * Use new import-each mode in casetest
 * Updated assign-slice tests
 - Removed object.valueOf() which allows transparent JS-ification of
   values. Unfortunately, it is not supported in IE... *sigh*
 * Change invocations of PY$__create__ and PY$__call__ to use "null"
   as the object, instead of {}.
 * Allow negative step values in tuple/list.__getitem__