use decimal type for arithmetic functions

[jennifersp]

fixes dolthub/dolt#4269

sum will be either float64 or decimal.Decimal type, but return type is decided on the type of the child expression of sum expression. This causes some test results to be modified into int from float.

the test results in decimal.Decimal type is hard to get the exact matching using its convert method, so result is checked in string format by using StringFixed() method of the decimal.Decimal result value.

[fulghum]

Looks good overall; I just had a few questions I wanted to make sure I understood.

I suspect that AVG needs the same treatment as SUM. We can handle that in a separate PR, but would be good to check that out while we're in here.

[fulghum]

Changes look good. This turned out to be trickier than I was expecting, so thanks for all the extra attention to detail. 😁