MINOR: stats: use stricter naming stats/field/line

MINOR: stats: use stricter naming stats/field/line

Several unique names were used for different purposes under statistics
implementation. This caused the code to be difficult to understand.

* stat/stats name is removed when a more specific name could be used
* restrict field usage to purely refer to <struct field> which
  represents a raw stat value.
* use "line" naming to represent an array of <struct field>