run examples and tests in parallel

[mikestillman]

It would be good if installPackage could run its M2 examples in parallel. Similarly for the tests from check Package.

Inside of Macaulay2, we probably have a way of seeing many processors we think we have. We could use that number of concurrent processes by default?

[mahrud]

#1454 goes 90% of the way in doing this, except that various functions in the Core are not reentrant; e.g. they use global variables that would be overwritten by concurrent invocations of the same function.