crypto: add utility hexhash functions

vlib/crypto/md5/md5.v

@@ -49,7 +49,7 @@ fn (d mut Digest) reset() {
 }
 
 // new returns a new Digest (implementing hash.Hash) computing the MD5 checksum.
-pub fn new() *Digest {
+pub fn new() &Digest {
 	mut d := &Digest{}
 	d.reset()
 	return d
@@ -141,3 +141,5 @@ fn block(dig mut Digest, p []byte) {
 pub fn (d &Digest) size() int { return Size }
 
 pub fn (d &Digest) block_size() int { return BlockSize }
+
+pub fn hexhash(s string) string { return sum(s.bytes()).hex() }