Use data.table backend

[gowerc]

Closes #117

[gowerc]

@kieranjmartin - Added an additional test and converted to full pr

[github-actions]

Code Coverage Summary

Filename                Stmts    Miss  Cover    Missing
--------------------  -------  ------  -------  --------------------------
R/ascii_tables.R          114       8  92.98%   10, 154, 164, 169-172, 231
R/cast_variables.R         49       0  100.00%
R/diffdf.R                201      18  91.04%   356-373, 400
R/generate_keyname.R       10       1  90.00%   16
R/identify.R              156       8  94.87%   287-294
R/is_different.R           52       0  100.00%
R/issuerows.R              40       0  100.00%
R/issues.R                 17       1  94.12%   51
R/misc_functions.R         34       2  94.12%   9, 13
R/print.R                  20       0  100.00%
TOTAL                     693      38  94.52%

Results for commit: 208b145

Minimum allowed coverage is 80%

♻️ This comment has been updated with latest results

[github-actions]

Unit Tests Summary

  1 files   13 suites   7s ⏱️
 55 tests  53 ✅ 2 💤 0 ❌
589 runs  581 ✅ 8 💤 0 ❌

Results for commit 208b145.

[kieranjmartin]

I think this should be held off for version 2. I think we might be building legacy we might want to remove.