Refactor spec.go to remove UnmarshalYAML boilerplate (#161)

Before this, each UnmarshalYAML function was a copy-pasted block of
20-ish lines of boilerplate. With this change, each UnmarshalYAML
function turns into a one-liner which calls unmarshalPlain.

This requires a somewhat regrettable amount of reflection code, but it's
arguably no more confusing than what was there before, while having the
benefit of being more concise and DRY.

Fixes #30.

I'm attempting to use stacked PRs here; I intend to switch the base
branch of this PR to `main` once the `drevell/for-each-values-from`
branch is merged.

templates/commands/render_action_include.go

@@ -53,7 +53,7 @@ func actionInclude(ctx context.Context, inc *model.Include, sp *stepParams) erro
 			return err
 		}
 
-		walkRelPath, err = safeRelPath(inc.Pos, walkRelPath)
+		walkRelPath, err = safeRelPath(&inc.Pos, walkRelPath)
 		if err != nil {
 			return err
 		}