Spacer variable refactor

[shawnbot]

This refactors our spacer variables so that the $spacer-{1-6} variables are the "source of truth", and $spacers just lists those values. This was awkward before with $spacer-1: nth($spacers, 1), and primer-marketing-support does it this way, so consistency++.

There's also a new variable, $spacer-map, which dramatically simplifies looping over the spacing scale:

// before, ugh
@for $i from 1 through length($spacers) {
  $size: nth($spacers, $i); // sm, md, lg, xl	
  $scale: $i - 1;  // 0, 1, 2, 3, 4, 5, 6
  .p-#{$scale} { padding: $size !important; }
}

// after, yay
@each $scale, $size in $spacer-map {
  .p-#{$scale} { padding: $size !important; }
}

Marketing spacer values ($spacer-{7..12}) also have !default now so they can be overridden.

This should not be a breaking change because the old variables still exist and their values are the same.