Inconsistent results of tableaux tcopy and shallow_tcopy

[nnposter]

Function tableaux.tcopy produces a full/deep copy of a given table, while tableaux.shallow_tcopy produces a shallow copy.

A user might naturally assume that results from these two functions are identical for flat tables (tables that do not contain nested tables). However, this is not guaranteed. The reason is that the two functions iterate over the table differently: tcopy is using pairs, while shallow_tcopy is using next. When pairs is used, the actual iterations are determined by a __pairs metamethod, not necessarily relying on next.

This PR updates tableaux.tcopy to use next to achieve consistency with shallow_tcopy. It will be merged in after March 1st unless concerns are raised.

[nnposter]

As a side note, construct

for k, v in next, tbl do
  ...
end

appears to iterate over large tables about 2.5x faster than

local k, v = next(tbl)
while k ~= nil do
  ...
  k, v = next(tbl, k)
end