hashsum: don't copy input buffer on Windows

Remove a copy operation of the input buffer being read for digest when
reading in text mode on Windows. Previously, the code was copying the
buffer to a completely new `Vec`, replacing "\r\n" with "\n". Instead,
the code now scans for the indices at which each "\r\n" occurs in the
input buffer and inputs into the digest only the characters before the
"\r" and after it.

Cargo.lock

@@ -2392,6 +2392,7 @@ dependencies = [
  "hex",
  "libc",
  "md5",
+ "memchr 2.4.0",
  "regex",
  "regex-syntax",
  "sha1",