builtin: x.vstring() instead of string(x) (#6102)

vlib/builtin/string.v

@@ -120,6 +120,23 @@ pub fn tos_lit(s charptr) string {
 	}
 }
 
+// byteptr.vstring() - converts a C style string to a V string. NB: the string data is reused, NOT copied.
+[unsafe]
+pub fn (bp byteptr) vstring() string {
+	return string{
+		str: bp
+		len: unsafe {C.strlen(bp)}
+	}	
+}
+
+// byteptr.vstring_with_len() - converts a C style string to a V string. NB: the string data is reused, NOT copied.
+[unsafe]
+pub fn (bp byteptr) vstring_with_len(len int) string {
+	return string{
+		str: bp
+		len: len
+	}	
+}
 
 // string.clone_static returns an independent copy of a given array
 // It should be used only in -autofree generated code.
@@ -1429,9 +1446,10 @@ pub fn (s string) repeat(count int) string {
 		}
 	}
 	unsafe {
-		ret[s.len * count] = 0
+		new_len := s.len * count    
+		ret[new_len] = 0
+		return ret.vstring_with_len(new_len)
 	}
-	return string(ret)
 }
 
 pub fn (s string) fields() []string {
@@ -1463,7 +1481,7 @@ pub fn (s string) filter(func fn(b byte) bool) string {
 	}
 	unsafe {
 		buf[new_len] = 0
-		return string(buf, new_len)
+		return buf.vstring_with_len(new_len)
 	}
 }
 
@@ -1524,6 +1542,6 @@ pub fn (s string) strip_margin_custom(del byte) string {
 	}
 	unsafe {
 		ret[count] = 0
+		return ret.vstring_with_len(count)
 	}
-	return string(ret)
 }