New class EccOpsTable to support the new concatenate-via-relations approach to the Merge protocol.

Background: The "Merge" protocol is essentially a protocol for establishing that a large table was constructed as the concatenation of smaller ones (subtables). In the original version of the protocol, the larger table was obtained by successively appending subtables (one from each circuit). The new version requires that subtables be PRE-pended. This results in a simpler protocol overall (and, importantly, one that's easier to make ZK) but its a bit more annoying from a data management standpoint since in general we don't want to pre-pend things in memory. This PR introduces a class EccOpsTable which stores individual subtables which can be virtually "prepended" to construct the entries of the corresponding aggregate table as needed.