* Create a simple Xamarin.Utils.Execution class that can handle all our
process execution needs:
* Captures or streams stdout/stderr (in UTF8).
* Supports async
* Supports a timeout
* Does not depend on any other source file we have, only uses BCL API.
* Have the execution helper classes from mtouch/mmp
(Xamarin.BundlerDriver.RunCommand) and the tests
(Xamarin.Tests.ExecutionHelper) use this new class.
* Some simplifications were made:
* All API that took a string array for the environment now takes a
Dictionary<string, string>.
* The Driver.RunCommand methods were split out to a separate file. This
file also contains a Verbosity field, which is conditioned on not being
in mtouch nor mmp, which makes including this file from other projects
simpler (such as bgen - in particular bgen was modified to use this
Verbosity field instead of its own).