Replace TOP-LEVEL-ENVIRONMENT with USER-INITIAL-ENVIRONMENT.
Pass mandatory environment arg to EVAL when SCHEME=book.mit
Add format.scm and pp.scm.
Set SHELL explicitly for mit-scheme's run-shell-command.
Define SYSTEM proc for book.mit.
Don't use uppercase letters in gensym6f.

Mit-scheme's string->symbol will wrap the symbol name in pipe chars if
the string contains uppercase letters. Symbols that contain pipe chars
are not valid C identifiers, so just pass lowercase strings to gensym6f.