Remove irrelevant internals from the public surface area

[abelbraaksma]

Certain functions, like Debug.xxx or the module TaskSeqInternals are publicly visible (though most of the actual functions are not). Likewise, types like TaskSeqResumptionFunc should probably be hidden.

Adding proper fsi files (currently only for the TaskSeq module), should do the trick.

[abelbraaksma]

Fixed in #111, will be published as part of v0.3.0.