Minor code clean-up

Apply most clippy code improvement suggestions. Ignore the
unnecessary_unwrap lint where non-beneficial (see rust-lang/rust-clippy#4530).

src/hexagon.rs

@@ -180,6 +180,7 @@ impl FloatCoordinate {
         -self.q - self.r
     }
 
+    #[allow(clippy::many_single_char_names)]
     pub fn from_pixel(layout: &Layout, p: Point) -> Self {
         let o = &layout.orientation;
         let x = (p.0 - layout.origin.0) / layout.size.0;
@@ -462,7 +463,7 @@ impl<'de> Deserialize<'de> for Direction {
             where
                 E: de::Error,
             {
-                if value >= 0 && value <= 5 {
+                if (0..6).contains(&value) {
                     Ok(Direction::from(value as u8))
                 } else {
                     Err(E::custom(format!("direction out of range: {}", value)))
@@ -473,7 +474,7 @@ impl<'de> Deserialize<'de> for Direction {
             where
                 E: de::Error,
             {
-                if value <= 5 {
+                if (0..6).contains(&value) {
                     Ok(Direction::from(value as u8))
                 } else {
                     Err(E::custom(format!("direction out of range: {}", value)))

src/tile.rs

@@ -514,7 +514,7 @@ impl FromStr for Terrain {
     type Err = ParseTerrainError;
 
     fn from_str(s: &str) -> Result<Self, Self::Err> {
-        let idx = s.find('-').unwrap_or(s.len());
+        let idx = s.find('-').unwrap_or_else(|| s.len());
         let surface = &s[0..idx];
 
         let level = if let Some(val) = s.get(idx+1..) {

src/view/export.rs

@@ -234,7 +234,7 @@ impl ExportView {
         // remove tile from todo list
         self.images.retain(|img| img.tile != *tile);
 
-        if self.images.len() == 0 {
+        if self.images.is_empty() {
             debug!("export of all tile images is completed");
             let url = check!(self.canvas.to_data_url_with_type("image/png").ok());
             check!(self.anchor.set_attribute("href", &url).ok());