Speed up long stack traces

[ChrisCinelli]

I am still using this puppy... and I am still an happy customer.

I was looking at the implementation of the long stack traces. It looks like that you do string concatenations for each async stack.

I was wondering... Would not be better to just store a reference to the string and lazy concatenate the strings only when somebody access to the err.stack ? I think you can save a lot of CPU and memory this way.