Use a plain Scheme file for the prelude

[acieroid]

Currently, the Scheme prelude is implemented as a map from names to their definitions (https://github.com/softwarelanguageslab/maf/blob/master/code/shared/src/main/scala/maf/language/scheme/primitives/SchemePrelude.scala). There is a duplication for the name (used once in the key of the map, once in the define part), and it's easy to mistype a name once.

Instead, if we could take the prelude from a plain .scm file, which can be assumed to have only a list of definitons, that could be avoided.
This should be a relatively simple change.