Column reordering: only move each col once per ALTER

[evanelias]

As described in https://github.com/skeema/skeema/issues/36#issuecomment-418090748, there are column-reordering situations in which Table.Diff() may attempt to reposition the same col multiple times, generating illegal SQL. One such case is covered by a new test in 1272a54a311; this hasn't been merged to master yet since the test currently fails.

The root cause is the current algorithm for handling column reorders doesn't guarantee that a column is only moved once. A different algorithm entirely should be used here. I believe this approach would provide the right guarantee:

• Among the common columns between the "before" and "after" side of the diff, identify the longest pre-sorted subsequence in the "before" side using an LIS (or, equivalently for this usage, LCS) algorithm. These columns do not need to be moved.
• All other columns do need to be moved, so emit exactly one MODIFY COLUMN ... AFTER|FIRST for each.
• Any non-moved columns should have a MODIFY COLUMN if (and only if) they had other modifications unrelated to ordering.

In theory, the optimal approach (in terms of minimum number of MODIFY COLUMN clauses) would also break ties in the first step by examining the number of other non-reorder modifications in each tied longest-subsequence candidate, to minimize the number of clauses generated in the third step. But that may wait until a future revision.

[evanelias]

Main issue fixed in bca198ed45475, and verified with new test coverage in 1272a54a311a and a41ba075709a8ef4.

Punting on the optimal tie-breaker logic until a future release, since this doesn't actually impact functionality and would greatly complicate the algorithm.