add tests in lwtreslib

Closed, please see MRs listed below

Because this MR's review and development was stuck/taking time, it has been split into smaller MR:

• !2243 (merged) contains miscellaneous changes unrelated to lists but necessary nontheless
• !2244 (merged) adds tests for Set and tests (and a fix) for Seq. This is not related to List, but it is related to Lwtreslib and also it sets the stage for the List tests later. (In fact, there is some dead-code introduced because the test support code is too generic for just Seq and Set and already has some bits that will be used for List)
• !1967 (closed) adds the List module (including tests)
• !2245 (merged) uses the List module

Note: !1967 (closed) and !2245 (merged) are meant to be merged together, they are split to ease the review. !2245 (merged) 's cdescription contains a link to inspect the diff of the two branches.