Rework day 1

Simplify part 2 a lot, by not actually computing the sums because they
do not matter, only the changes do. Also eliminate the allocation
overhead while parsing line-by-line input.

Fixes the existing clippy error because the offending line no longer
exists.

2021/src/common.rs

@@ -0,0 +1,45 @@
+use std::io::BufRead;
+use std::io::BufReader;
+use std::io::Read;
+use std::marker::PhantomData;
+use std::str::FromStr;
+
+/// Line-based iterator/parser
+///
+/// For each line of the input, attempt to parse it as the requested type. Iteration is stopped on
+/// the first IO error or parse error, silently. Leading and trailing whitespace is stripped before
+/// attempting to parse.
+pub struct LineParser<'a, I>
+where
+    I: FromStr,
+{
+    reader: BufReader<&'a mut dyn Read>,
+    buffer: String,
+    _data: PhantomData<I>,
+}
+
+impl<'a, I: FromStr> LineParser<'a, I> {
+    pub fn new(input: &'a mut dyn Read) -> Self {
+        Self {
+            reader: BufReader::new(input),
+            buffer: String::new(),
+            _data: PhantomData,
+        }
+    }
+
+    fn next_line(&mut self) -> Option<&str> {
+        self.buffer.clear();
+
+        self.reader.read_line(&mut self.buffer).ok()?;
+
+        Some(self.buffer.trim())
+    }
+}
+
+impl<'a, I: FromStr> Iterator for LineParser<'a, I> {
+    type Item = I;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.next_line()?.parse().ok()
+    }
+}