Improve handling of low-memory situations

[drumm]

Reported at https://www.drupal.org/project/infrastructure/issues/3477553#comment-15846839:

I also had problems with installation and now get out of memory errors when running composer up in both ddev and on my MacOS host.
PHP Fatal error: Allowed memory size of 1610612736 bytes exhausted (tried to allocate 20480 bytes) in /var/www/html/vendor/php-tuf/php-tuf/src/CanonicalJsonTrait.php on line 46

1.6MB is not a high memory limit. Whatever is usually necessary would be good to have in a friendly error message before getting to a fatal error.

If there are opportunities to reduce memory use, that is good to implement, too.