Add DecimalBaseColumn and move as_decimal_column

[isVoid]

It is foreseeable that a few routines can be shared across Decimal32Column and Decimal64Column, thus I added a DecimalBaseColumn so that we may move shared routines to this base column. In this PR, I moved as_decimal_column so as to close #8853 .

[galipremsagar]

rerun tests

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (branch-21.10@f6ce668). Click here to learn what that means.
The diff coverage is n/a.

❗ Current head 9312571 differs from pull request most recent head c2a6286. Consider uploading reports for the commit c2a6286 to get more accurate results

@@               Coverage Diff               @@
##             branch-21.10    #9001   +/-   ##
===============================================
  Coverage                ?   10.58%           
===============================================
  Files                   ?      116           
  Lines                   ?    19062           
  Branches                ?        0           
===============================================
  Hits                    ?     2017           
  Misses                  ?    17045           
  Partials                ?        0           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f6ce668...c2a6286. Read the comment docs.

[galipremsagar]

@gpucibot merge