"Mahayana" is a Sanskrit word composed of "maha" for "great" and "yana" for "vehicle." It's used to describe a diverse tradition of Buddhism that somehow (the history is not exactly clear) diverged from that monastic consensus which became the Theravada ("way of the elders") tradition...

Mahayana sees itself as going beyond what they might call "individual" or "personal" liberation, in that it sets up a grand goal of liberating all beings. Practitioners hold this ambition in mind, and see the "Hinayana" (small vehicle; a pejorative term used in Mahayana rhetoric to denounce other interpretations of Buddhism) ambition as limited or as merely a first step towards the ultimate goal of universal liberation.

How this relates to the Haskell prelude, I'm not so sure... but yeah, I suppose a "Mahayana prelude" is more grandly ambitious and therefore also more expansive and difficult to realize...

(By the way, in religious studies, Rupert Gethin's book The Foundations of Buddhism has an introductory chapter about Mahayana, and an in depth review is Paul Williams's Mahayana Buddhism: the Doctrinal Foundations.)

So the terms are coming from the Buddhist tradition, where there is a category of Buddhist denominations usually (a) valuing the Lotus Sutra (as sort of a central uniting point and/or shibboleth); (b) believing in celestial buddhas and bodhisattvas who you venerate and whose assistance you request in life; (c) valuing the idea of us all being "on the path to nirvana" (the "bodhisattva" ideal) even if we are not monks; and (d) representing the diversity of Buddhist denominations as largely being "lesser vehicles" (hinayana) to nirvana, and their own category as being a "greater vehicle" (mahayana) to get there. "Maha-" is a prefix meaning "big," but often also connoting "great, impressive, respectable, noble".

In practical terms a "big-vehicle Prelude" is saying "I want to do something much smarter than the original Prelude and I am willing to replace most of the underlying abstractions in the Prelude to do it," which can sometimes mess with code that has to deal with old-prelude and new-prelude subsystems of code. A "small-vehicle Prelude" is much more "I have a couple extra modules here and there that I want to import, and maybe I want to fix a couple of these operators to a slightly more general definition with a typeclass, but largely I want everything to be the same, hence intercompatible with minimum fuss."

Sorry, dumb joke. Mahayana Buddhism is also known as "big vehicle" or "great vehicle" Buddhism.