lib/std/io/stream: add LimitReader (#858)

* lib/std/io/stream: add LimitReader

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

* lib/std: more method conversions to use new receiver notation

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

---------

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

lib/std/io/stream/limitreader.c3

@@ -0,0 +1,36 @@
+module std::io::stream;
+
+struct LimitReader
+{
+    Stream reader;
+    usz limit;
+}
+
+fn void LimitReader.init(&self, Stream reader, usz limit)
+{
+    *self = { .reader = reader, .limit = limit };
+}
+
+fn usz! LimitReader.read(&self, char[] bytes)
+{
+    if (self.limit == 0) return IoError.EOF?;
+    usz m = min(bytes.len, self.limit);
+    usz n = self.reader.read(bytes[:m])!;
+    self.limit -= n;
+    return n;
+}
+
+fn Stream LimitReader.as_stream(&self)
+{
+    return { .fns = &limitreader_interface, .data = self };
+}
+
+fn usz LimitReader.available(&self)
+{
+    return self.limit;
+}
+
+StreamInterface limitreader_interface = {
+    .read_fn = fn(s, char[] bytes) => ((LimitReader*)s.data).read(bytes),
+    .available_fn = fn(s) => ((LimitReader*)s.data).available(),
+};