Extensions module renamed to khash-extensions

[RiccardoM]

Warning: Should be merged after GH-8.

Changes

• Renamed the extensions module to khash-extensions.
  Being this project inside the group org.komputing, having a generic extension module could have lead to conflicts between other projects inside the same group and with a module named the same way if both libraries were to be published on Maven
• Updated the konitnuum.json file to execute the check task before the build one.
  The check task is the replacement of the text task inside multiplatform projects.

[RiccardoM]

I've just pushed a new commit that contains the renaming of all the packages, prepending the org.komputing prefix to all of them. This was due in order to have consistency with other Komputing projects.