Maven fix and small generic-type improvement.

I couldn't get the library to compile via Maven as it didn't pick up the sources. The pom.xml file still referred to the src/main directory whereas it should be core/src/main. Also, don't know if the build-maven-helper-plugin is needed as I could set the source-directory in the build tag.

I also made some slight changes to the formal type parameters for some of the constructor methods of Array, List, Set and Stream. This basically follows the PECS convention to allow for easier and more flexible use of said methods. Without the widened type parameters I had to do a lot of casting of the sort:

class A { }

class B extends A { }

Iterable\<B\> iterable = ...;
F\<A, C\> func = ...;
Stream\<A\> stream = (Stream)iterableStream(iterable); // <--
Stream\<C\> result = stream.map(func);