Optimize performance of call-stack getting

[Gvozd]

call stack not actually needed at most cases, but got always
get(actually only first get) call stack from error is so slow operation, and may be moved on-demand getter

Purpose (TL;DR) - mandatory

Optimize work with callstack of stubbing
Call of stubbed functions was 5-15 times faster

Background (Problem in detail) - optional

When stubbed function call, that remember hist call stack for use in call.toString()
But get stack-property from Error-object is so slow in first getting
Also this very slow with babel, because used source-map convert of stack
I think, that not actually need get stack, when call-object constructed, but may getted when this really need(at toString() method)

Solution - optional

I not resolve stack at call-object constructring, but I remeber only generated Error-object
Only when stack actually needed, I get it from Error-object
Important: only first getting of stack from Error-object is slow. Repeated get stack is cached(I check this at Chrome)

How to verify - mandatory

1. Check out this branch (see github instructions below)
2. npm install
3. Put in root of repo this file https://gist.github.com/Gvozd/291e4f0adf247ff70ed72150059e4977
4. Run this file with node or babel-node command
5. Performance of running this example is better, than withount my fix
   x5 times at Node.js
   x15 times at babel-node

[coveralls]

Coverage remained the same at 95.421% when pulling a865362 on Gvozd:optimize_performance into 46e41c8 on sinonjs:master.

[coveralls]

Coverage remained the same at 95.421% when pulling 6b27ce4 on Gvozd:optimize_performance into 46e41c8 on sinonjs:master.

[fatso83]

This seems good to me. Any objections?

[cjohansen]

👍 Nice work!