Merge enforceEx with enforce

Makes it possible to call `enforce!MyException` instead of `enforceEx!MyException`, eliminating
redundant notation. Old enforceEx is not deprecated but removed from documentation examples.

This fixes one of very common Phobos "WAT" moments for me - having separate name for semantically identical
function with a different argument set.
I believe we had it like this at one point, though, so it's not a completely outlandish suggestion. ;)