Inconsistent component default/spacer padding

Created by: bbugh

We've been running into consistency issues using Bootstrap 4beta2's default spacing.

Most of the components ($table-cell-padding, $card-spacer-y, $alert-padding-y, etc.) use a padding: .75rem. However, the $spacers don't include that value. It's not possible to use a built in padding class to match the components, which is very weird.

For example, putting a table into a card:

<div class="card">
  <h4 class="card-header">My Header</h4>
  <table class="table">
    <tr> 
      <td>Label</td>
      <td>Value</td>
    </tr>
  </table>
</div>

Results in HTML that looks like this:

The .card-header class is using a hard-coded 1.25rem padding (from $card-spacer-x), while the table cell is using hard-coded .75rem (from $table-cell-padding).

I would expect to be able to have some kind of pl-? (left) or pt-? (top) class that matches these components default, since it's in so many places. However, there aren't any Bootstrap-defined classes that you can use to get that .card-header padding to match all of the default component paddings. I realize we can make our own class, but it seems like an oversight and an actual issue to note.