Optimize `scm_read_string'.

Optimize `scm_read_string'.

According to the new benchmarks, this leads a 5% speed improvement when
reading small strings, and a 27% improvement when reading large strings.

* libguile/read.c (READER_STRING_BUFFER_SIZE): Change to 128; update
  comment to mention codepoints.
  (scm_read_string): Make `str' a list of strings, instead of a string.
  Store characters read in buffer `c_str'.  Cons to STR when C_STR is
  full, and concatenate/reverse at the end.

* benchmark-suite/benchmarks/read.bm (small, large): New variables.
  Set %DEFAULT-PORT-ENCODING to "UTF-8".
  ("read")["small strings", "large strings"]: New benchmarks.