Bug 984241: Refactor/optimize grid-template-areas, store number of columns. r=dholbert
In the parsing code for grid-template-areas:

* Move some of the processing into the helper function,
  so that the grid-template will not have to duplicate it.

* Store the number of parsed columns (and rows),
  as they will affect the size of the "explicit grid".

* Use a hash table instead of a linear scan to avoid O(n²)-ish complexity.