Correct names of layers that have unclear names for historical reaons

Correct names of layers that have unclear names for historical reasons:
- Rename #roads to #roads-low-zoom
- Rename #minor-roads-casing to #roads-casing
- Rename #minor-roads-fill to #roads-fill

This pull request solves the following problem:
- The layers #roads and #minor_roads are confusingly named.

This pull request changes the rendering as follows:
- No change.