Support Inform7's basic types

[MarcCote]

Currently, TextWorld only supports Inform7's things but it would be better if it was also supporting other types like number and text. For instance, it would allow us to create safe objects (kind of container) that have a lock combination (a number or maybe some text). For a concrete example see https://github.com/Microsoft/TextWorld/compare/master...MarcCote:enh_safe#diff-177ef43219a0bd25919149e6e57f22b6R229.