add reader option for parsing [] as ().

add reader option for parsing [] as ().

* libguile/private-options.h:
* libguile/read.c (scm_read_opts, SCM_SQUARE_BRACKETS_P): Add an option
  for treating [ and ] as parentheses, on by default. Note that this
  makes them delimiters also, so [ and ] cannot appear in a symbol name,
  with this read option on.
  (scm_read_sexp): If we start with [, we end with ].
  (scm_read_expression): Add case for [.