Allow to access a Memory with more than 32767 pages (#166)

* Add new Int64-based Memory APIs to allow to access memories with more than 32767 pages.

Fixes #165

* Follow-Up: Fix superfluous entry.

* PR feedback: Remove the int-based overloads which have been replaced with long-based ones.

* PR feedback: Fix style and missing comment.

* Use <para> in the <summary> element to better structure paragraphs.

Author: kpreisser