refactor: split and rename column_operation.rs (#343)

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# Rationale for this change
`column_operation.rs` is incredibly long. It is time to split out
`ColumnType` manipulations as a separate file. Moreover since we will
most likely have a separate file for `Column` operations and that we
don't even use either `OwnedColumn` or `Column` in `column_operation.rs`
it is a misnomer and should be renamed `slice_operation.rs`.
Decimal-related code is long, has unusual dependencies and can't easily
be simplified hence they should be split into a separate file too.
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 Add `NestedLoopJoinExec`.
 Closes #345.

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# What changes are included in this PR?
- Move `ColumnType`-related code to `column_type_operation.rs`
- Move decimal-related slice manipulation code to
`slice_decimal_operation.rs`
- Rename the file to `slice_operation.rs`
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