move all vlib modules to vlib/

vlib/builtin/string_builder.v

@@ -0,0 +1,39 @@
+// Copyright (c) 2019 Alexander Medvednikov. All rights reserved.
+// Use of this source code is governed by an MIT license
+// that can be found in the LICENSE file.
+
+module builtin
+
+struct StringBuilder {
+	buf []byte
+	len int
+}
+
+pub fn new_string_builder(initial_size int) StringBuilder {
+	return StringBuilder {
+		buf: new_array(0, initial_size, sizeof(byte))
+	}
+}
+
+pub fn (b mut StringBuilder) write(s string) {
+	b.buf._push_many(s.str, s.len)
+	b.len += s.len
+}
+
+pub fn (b mut StringBuilder) writeln(s string) {
+	b.buf._push_many(s.str, s.len)
+	b.buf << `\n`
+	b.len += s.len + 1
+}
+
+pub fn (b StringBuilder) str() string {
+	return tos(b.buf.data, b.len)
+}
+
+pub fn (b StringBuilder) cut(n int) {
+	b.len -= n
+}
+
+pub fn (b mut StringBuilder) free() {
+	C.free(b.buf.data)
+}