Modifying string returned from symbol->string shouldn't affect original symbol

[Takashi Kato]

This shouldn't return 'b

(define s (string #\a))
(define sm (string->symbol s))
(string-set! (symbol->string sm) 0 #\b)
(print sm)

[Takashi Kato]

Making copied string for symbol literal (Fixes #168)

→ <<cset 0cd94e6fabe4>>