Add support for BigDecimal 4.x

[schinery]

Some changes to allow support for BigDecimal 4.x

• Add support for BigDecimal 4.x while maintaining support for >= 3.1
• Add Appraisal gem and update CI to test BigDecimal 3 and 4 in supported Ruby versions

This would resolve #401.

NOTE: I had trouble getting geth --dev --http --ws --ipcpath /tmp/geth.ipc & to run locally, so I had a number of spec fails. Hopefully these will be fine on the CI and not because of the BigDecimal bump.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.22%. Comparing base (87cafff) to head (e54a9af).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #402   +/-   ##
=======================================
  Coverage   99.22%   99.22%           
=======================================
  Files          91       91           
  Lines        6829     6829           
=======================================
  Hits         6776     6776           
  Misses         53       53           

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[l5yth]

Thanks, give me a couple of days to process this.