Refactored out most of internal work for summarizing data opening
the door for generating charts from it. A model is introduced
to hold information needed for a summary, Histogram command is
an example of a partial usage. This is the beginning.
Removed implicit arithmetic traits on Value and Primitive to avoid
mixed types panics. The std operations traits can't fail and we
can't guarantee that. We can handle gracefully now since compute_values
was introduced after the parser changes four months ago. The handling
logic should be taken care of either explicitly or in compute_values.
The zero identity trait was also removed (and implementing this forced
us to also implement Add, Mult, etc)
Also: the `math` operations now remove in the output if a given column is not computable:
```
> ls | math sum
──────┬──────────
size │ 150.9 KB
──────┴──────────
```
* add variance (population)
subcommand to math
* impl variance subcommand with spanning errors for invalid types
* add stddev subcommand to math
* rename bytes to filesize
* clippy fix -- use expect instead of unwrap in variance tests
