Working on REPL

If sagittarius does not get any script file to process, then it will go in to REPL (read-eval-print-loop). For developers' convenience, REPL imports some libraries by default such as (rnrs).

If .sashrc file is located in the directory indicated HOME or USERPROFILE environment variable, then REPL reads it before evaluating user input. So developer can pre-load some more libraries, instead of typing each time.

NOTE: .sashrc is only for REPL, it is developers duty to load all libraries on script file.