Upgraded to ScalaTest 3.0.8.

Now cross-compiles against Scala 2.13.0.

Bumped ending copyright year to 2019

Implemented a better way to handle compatibility between Scala 2.13 a…

…nd earlier versions, one that requires less duplicate code, by providing appropriate type aliases in a new, conditionally-compiled grizzled.ScalaCompat package. Functions that use Stream in 2.12 and earlier still use LazyList in 2.13, but the implementation is better. Bumped the minor version, rather than the patch version, even though there aren't major API changes, because of the possible impact on compilation.

grizzled.file.util.listRecursively() and grizzled.file.Implicits.Griz…

…zledFile.listRecursively() now return LazyList for Scala 2.13. They still return Stream for Scala 2.12 and earlier.

Various internal functions that used Stream have been tweaked to use LazyList for 2.13.

These changes necessitated some refactoring, as well as the introduction of src/main/pre-scala-2.13 and src/main/scala-2.13 directories to the source code, to contain parallel implementations.

Fixed merge conflicts.

4.6.0, for Scala 2.10 only.

Change /: to foldLeft, since /: is deprecated in Scala 2.13. (Thanks to 

@xuwei-k for the patch.)

Change /: to foldLeft, since /: is deprecated in Scala 2.13. (Thanks to 

@xuwei-k for the patch.)

Re-enabled PGP plugin.

Changed grizzled.datetime.EnhancedDuration to grizzled.datetime.Enric…

…hedDuration, for consistency.