* Sample command
* Join command with checks
* More dataframes commands
* Groupby and aggregate commands
* Missing feature dataframe flag
* Renamed file
* New commands for dataframes
* error parser and df reference
* filter command for dataframes
* removed name from nu_dataframe
* commands to save to parquet and csv
This commit extracts Tag, Span, Text, as well as source-related debug
facilities into a new crate called nu_source.
This change is much bigger than one might have expected because the
previous code relied heavily on implementing inherent methods on
`Tagged<T>` and `Spanned<T>`, which is no longer possible.
As a result, this change creates more concrete types instead of using
`Tagged<T>`. One notable example: Tagged<Value> became Value, and Value
became UntaggedValue.
This change clarifies the intent of the code in many places, but it does
make it a big change.
