Coalton readtable consumes everything from a stream w/o existing metadata

[shirok]

Related to #1706

This may be by design. However, if you want to read multiple sexpr from a stream, it may interferes it.

CL-USER> *readtable*
#<NAMED-READTABLE COALTON:COALTON {1200BC08C3}>
CL-USER> (with-input-from-string (s "(a)(b)") (list (read s nil nil) (read s nil nil)))
((A) NIL)

If I'm using Slime and I run :coalton/tests, I'm in this state.

This is because read-coalton-toplevel-open-paren copies the entire input; the input stram is exhausted when the first call of read returns.

        (t
         ;; no metadata, no file operation, therefore we are in a
         ;; repl: bind a source-string containing cloned input
         (let ((*source* (coalton-impl/source:make-source-string
                          (with-output-to-string (out)
                            (write-char #\( out)
                            (alexandria:copy-stream stream out))
                          :name "repl")))
           (with-open-stream (stream (source:source-stream *source*))
             (read-char stream)
             (maybe-read-coalton stream *source*))))))

• If the Coalton readtable isn't supposed to be used when the program uses read other than reading Coalton source, this may be a non-issue. We need to figure out why Slime uses Coalton readtable while executing CL code, though.
• If the Coalton readtable is supposed to be transparent (e.g. it should function just like normal readtable if auxiliary info isn't there), we may need to do something.

[stylewarning]

This is a good find and I don't have an immediately good answer. The main use is to get line-col info. (Don't make fun of us Common Lispers and our weak idea of syntax...) We want to make sure we can always get that in "normal" use.