micro-optimizations to string-trim-both, and to (web http)

micro-optimizations to string-trim-both, and to (web http)

* libguile/srfi-13.c (scm_string_trim, scm_string_trim_right)
  (scm_string_trim_both): Take the whitespace fast-path if the char_pred
  is scm_char_set_whitespace.

* module/web/http.scm (read-header, split-and-trim, parse-quality-list):
  (parse-param-component, parse-credentials, "Content-Type"):
  (read-request-line, read-response-line): Use char-set:whitespace
  instead of char-whitespace?.  It avoids recursing into the VM.