read: Support R7RS |...| symbol notation.

read: Support R7RS |...| symbol notation.

* libguile/private-options.h (SCM_R7RS_SYMBOLS_P): New macro.
  (SCM_N_READ_OPTIONS): Increment.

* libguile/read.c (scm_read_opts): Add entry for 'r7rs-symbols'.
  (t_read_opts): Add field for 'r7rs_symbols_p'.
  (scm_read_string_like_syntax): New function based on earlier
  'scm_read_string' that handles either string literals or R7RS quoted
  symbols (delimited by vertical bars), depending on the value of 'chr'.
  (scm_read_string): Reimplement based on 'scm_read_string_like_syntax'.
  (scm_read_r7rs_symbol): New static function.

* doc/ref/api-data.texi (Symbol Read Syntax): Briefly describe the R7RS
  symbol syntax, mention the 'r7rs-symbols' read option, and give some
  examples.

* doc/ref/api-evaluation.texi (Scheme Read): Mention the 'r7rs-symbols'
  read option.

* test-suite/tests/reader.test ("reading"): Add test.