vlib: add an encoding.xml module with parser, validation, entity encoding, unit tests (#19708)

vlib/encoding/xml/reader_util.v

@@ -0,0 +1,30 @@
+module xml
+
+import io
+
+fn next_char(mut reader io.Reader, mut buf []u8) !u8 {
+	if reader.read(mut buf)! == 0 {
+		return error('Unexpected End Of File.')
+	}
+	return buf[0]
+}
+
+struct FullBufferReader {
+	contents []u8
+mut:
+	position int
+}
+
+[direct_array_access]
+fn (mut fbr FullBufferReader) read(mut buf []u8) !int {
+	if fbr.position >= fbr.contents.len {
+		return io.Eof{}
+	}
+	remaining := fbr.contents.len - fbr.position
+	n := if buf.len < remaining { buf.len } else { remaining }
+	unsafe {
+		vmemcpy(&u8(buf.data), &u8(fbr.contents.data) + fbr.position, n)
+	}
+	fbr.position += n
+	return n
+}