Add bootstrap distributions to results

[davidskalinder]

Depends on #31, its dependency #30, and its dependency #26.

This is a small change to actually include all the bootstrap distributions in the results. This adds to the output of OaxacaBlinderDecomp() but doesn't change or remove anything.

Here is the old structure of results$bootstraps:

• overall
  • data frame with metrics (e.g., se, CI boundaries) as columns and term types (e.g., endowments etc.) as rows
• varlevel
  • data frame with metrics (e.g., se, CI boundaries) as columns and term-type/term pairs as rows

Here is the new structure:

• overall
  • data frame with metrics (e.g., se, CI boundaries) as columns and term types (e.g., endowments etc.) as rows
• varlevel
  • data frame with metrics (e.g., se, CI boundaries) as columns and term-type/term pairs as rows
• dists
  • overall
  • list of term types, each containing a data frame with the column "overall" and each bootstrap run as rows
  • varlevel
  • list of term types, each containing a data frame with term names as columns and each bootstrap run as rows

I do think that ultimately it'd be a little more elegant to have the distributions and the summaries at the same level, something like this:

• bootstraps
  • overall
  • summaries
  • distributions
  • varlevel
  • summaries
  • distributions

... or perhaps with the finest- and next-finest levels reversed. However this would break the existing hierarchy, so I've put the new $dists element alongside the old ones for now.

Nothing in the generics touch this yet, so this shouldn't affect users much unless they go looking for it.

A similar PR for the bootstrapped gaps is coming shortly.

@sinanpl, of course let me know if anything needs clarification!

[davidskalinder]

Oh, by the way, the motivation for this PR is from this stackexchange comment, which, in addition to recommending a zillion bootstrap runs, recommends looking at histograms of the bootstrap distribution to ensure that it looks "regular".

[davidskalinder]

As in https://github.com/sinanpl/OaxacaBlinder/pull/34#issuecomment-2065349954, this has been updated so that the name of the single column in each element of the overall bootstrap distribution list is "estimate" instead of "overall".