The compare() function now saves a shallow copy of the input tables. This
means functions using a comparison, e.g. slice_diffs(), now only need the
comparison argument, rather than requiring the original input tables to be
re-supplied.
New functions weave_diffs_wide() an weave_diffs_long() are added, which show
the differing values in context with other columns from the input tables.
A new function slice_unmatched() is added, used to get the rows from
input tables a comparison shows as not existing in the other table
A new function slice_diffs_both() is added, used to get the differences from
both input tables in a single output with the rows interleaved
The error message for duplicated by values now shows the row which is duplicated
Comparisons are now faster