Change alt/orig to build/to use heading levels

As the 'Alternate code' and 'Original code' heading level is now 2 the
'To build' and 'To use' subsections should be level 3, not 4, and also
reworded, this was done:

    sgit -e 's/^#### To build/### Alternate build/g' -e 's/^#### To use/### Alternate use/g' '*README.md'

Many more have to be updated and this will be done manually at another
time.

1989/tromp/README.md

@@ -53,7 +53,7 @@ loser](https://web.archive.org/web/20181023221954/https://www.chicagotribune.com
 The author provided an alternate version of the code with some improvements. See
 the judges' additional remarks in the author's remarks, for more details.
 
-#### To use:
+### Alternate use:
 
 ```sh
 make alt