Establish representation of Lesson objects
Establish representation of Lesson objects

Our most immediate goal is to create a facet which allows filtering modules
by Lecture Timings. This requires us to come up with a more compact
representation for a module's Timetable and the Lesson objects within it,
otherwise the data will take up too much size.

Signed-off-by: Pang Yan Han <22594c345857f664c61cd411f607c4d40a9d4af9@gmail.com>

yanhan/cljs-nusmods,yanhan/cljs-nusmods,yanhan/cljs-nusmods