This commit fixes a bug (or feature? unclear at this stage) of the
`rootOf` function inside `searchTableNgrams`, which could, in some
occasion, return a root which has a different `ListType`, which
in turn throws off pagination.

This commit adds some basic tests for the Ngrams pagination,
capitalizing on the refactoring of the `searchTableNgrams` function,
which is now pure and can be tested fairly easily.

This commit refactors the Ngrams pagination code to use
the `NgramsSearchQuery`, and it introduces a new function called
`searchTableNgrams`, which is now completely pure.