Abstract the core typography mixins into single core mixin

Making the codebase more DRY (don't repeat yourself).
Create a single core() mixin which accepts $font-size, $font-size-640 and $font-size-print in addition to the existing $line-height, $line-height-640 and $tabular-numbers.
Existing mixins call the new core base.
If you pass no arguments, you get core-19.
Dubious merit of allowing creation of new core font sizes.