Row selection unit tests

[martinblostein]

Suite of tests consisting of iterated simple row selections and permutations to make sure that the proxy_table updates correctly.

Along these lines:

dt <- data.table(x = 1:10)
ft <-fsttable::fst_table(fst::write_fst(dt, tempfile))
ft[2:6][3:2][1] # expect 4

("Subissue of #4)

[MarcusKlik]

Yes, perfect. And we can add a series of helper functions to compare a datatableinterface object with a second datatableinterface or an in-memory data.table (like this one).