Race on global variable: factorials

We get a RACE error on running tests in the production with the flag -race enabled. And the stack shows it comes from here:

I believe that the reason is multiple goroutines are calling method ExpTaylor and this method manipulates the shared variable factorials (https://github.com/shopspring/decimal/blob/master/decimal.go#L84). As you used a global variable for caching factorials (https://github.com/shopspring/decimal/pull/229), it's not goroutine safe and should have a lock.