refactor: address clippy warnings and improve code quality (#409)

- Fix string formatting with modern interpolation syntax
- Improve Debug implementation with finish_non_exhaustive()
- Fix function placement in shape.rs to avoid items_after_statements warning
- Use more idiomatic Rust patterns (map_or_else, next_back)
- Clean up conditional imports in vi.rs
- Convert multiple methods to `const` functions for optimization and consistency
- Introduce `core_maths` for enhanced no-std compatibility
- Update `Cargo.toml` for the new optional dependency and feature adjustments

src/cursor.rs

@@ -43,7 +43,7 @@ impl Affinity {
         *self == Self::After
     }
 
-    pub fn from_before(before: bool) -> Self {
+    pub const fn from_before(before: bool) -> Self {
         if before {
             Self::Before
         } else {
@@ -51,7 +51,7 @@ impl Affinity {
         }
     }
 
-    pub fn from_after(after: bool) -> Self {
+    pub const fn from_after(after: bool) -> Self {
         if after {
             Self::After
         } else {