Expose parse-string, parse-next and eval in core namespace

[borkdude]

This preliminary API is already exposed and open for feedback. Should be considered undocumented until docstrings are added. This API should make writing REPLs in web applications etc easier.

user=> (require '[sci.core :as sci])
nil
user=> (def ctx (sci/init {}))
#'user/ctx
user=> (def form (sci/parse-string ctx "(+ 1 2 3)"))
#'user/form
user=> form
(+ 1 2 3)
user=> (sci/eval ctx form)
6
user=> (def multiple-forms "(+ 1 2 3) (+ 4 5 6)")
#'user/multiple-forms
user=> (def reader (sci/reader multiple-forms))
#'user/reader
user=> (def next-form (sci/parse-next ctx reader))
#'user/next-form
user=> next-form
(+ 1 2 3)
user=> (sci/eval ctx next-form)
6
user=> (def next-form (sci/parse-next ctx reader))
#'user/next-form
user=> next-form
(+ 4 5 6)
user=> (sci/eval ctx next-form)
15
user=> (def next-form (sci/parse-next ctx reader))
#'user/next-form
user=> next-form
:sci.core/eof