Added updated task launch implementation that now tracks task groups.

Within each function that launches tasks, we now can easily track which
tasks that function launched, so that the sync at the end of the function
can just sync on the tasks launched by that function (not all tasks
launched by all functions.)

Implementing this led to a rework of the task system API that ispc generates
code to call; the example task systems in examples/tasksys.cpp have been
updated to conform to this API.  (The updated API is also documented in
the ispc user's guide.)

As part of this, "launch[n]" syntax was added to launch a number of tasks
in a single launch statement, rather than requiring a loop over 'n' to
launch n tasks.

This commit thus fixes issue #84 (enhancement to launch multiple tasks from
a single launch statement) as well as issue #105 (recursive task launches
were broken).

expr.h

@@ -250,7 +250,8 @@ public:
  */
 class FunctionCallExpr : public Expr {
 public:
-    FunctionCallExpr(Expr *func, ExprList *args, SourcePos p, bool isLaunch);
+    FunctionCallExpr(Expr *func, ExprList *args, SourcePos p, 
+                     bool isLaunch = false, Expr *launchCountExpr = NULL);
 
     llvm::Value *GetValue(FunctionEmitContext *ctx) const;
     const Type *GetType() const;
@@ -263,6 +264,7 @@ public:
     Expr *func;
     ExprList *args;
     bool isLaunch;
+    Expr *launchCountExpr;
 
 private:
     void resolveFunctionOverloads(bool exactMatchOnly);