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

vlib/strings/builder.js.v

@@ -38,7 +38,7 @@ pub fn (mut b Builder) writeln(s string) {
 }
 
 pub fn (b Builder) str() string {
-	return string(b.buf, b.len)
+	return unsafe { byteptr(b.buf.data).vstring_with_len(b.len) }
 }
 
 pub fn (mut b Builder) cut(n int) {

vlib/strings/builder.v

@@ -116,7 +116,7 @@ pub fn (mut b Builder) str() string {
 			'If you want to reuse a builder, call b.free() first.')
 	}
 	b.buf << `\0`
-	s := string(b.buf,b.len)
+	s := tos(b.buf.data, b.len)
 	bis := b.initial_size
 	//free(b.buf.data)
 	b.buf = []byte{cap: bis}

vlib/strings/strings.c.v

@@ -10,7 +10,7 @@ pub fn repeat(c byte, n int) string {
 		C.memset( bytes, c, n )
 		bytes[n] = `0`
 	}
-	return string( bytes, n )
+	return unsafe { bytes.vstring_with_len(n) }
 }
 
 // strings.repeat_string - gives you `n` repetitions of the substring `s`
@@ -34,5 +34,5 @@ pub fn repeat_string(s string, n int) string {
 	unsafe {
 		bytes[blen] = `0`
 	}
-	return string( bytes, blen )
+	return unsafe { bytes.vstring_with_len(blen) }
 }

vlib/strings/strings.js.v

@@ -4,9 +4,8 @@ pub fn repeat(c byte, n int) string {
 	if n <= 0 {
 		return ''
 	}
-	mut arr := [c].repeat(n + 1)
-	arr[n] = `\0`
-	return string(arr,n)
+	arr := [c].repeat(n)
+	return arr.bytestr()
 }
 
 pub fn repeat_string(s string, n int) string {