coord: ranged1d: fix irrefutable if let warning

I keep getting this annoying warning with rust 1.83:

warning: irrefutable `if let` pattern
   --> plotters/src/coord/ranged1d/types/numeric.rs:29:20
    |
29  |                 if let Ok(index) = Self::ValueType::try_from(index) {
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
365 | impl_discrete_trait!(RangedCoordusize);
    | -------------------------------------- in this macro invocation
    |
    = note: this pattern will always match, so the `if let` is useless
    = help: consider replacing the `if let` with a `let`
    = note: `#[warn(irrefutable_let_patterns)]` on by default
    = note: this warning originates in the macro `impl_discrete_trait` (in Nightly builds, run with -Z macro-backtrace for more info)

This is because the conversion of usize to ValueType always
passes, so we don't need the try_from().

The problem with this operation is not the conversion which
always succeeds, instead it is the potential overflows, so we
use checked_add() which returns None if the addition fails.

Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>

Author: 10ne1

plotters/src/coord/ranged1d/types/numeric.rs

@@ -1,4 +1,3 @@
-use std::convert::TryFrom;
 use std::ops::Range;
 
 use crate::coord::{combinators::WithKeyPoints, ranged1d::{
@@ -26,10 +25,8 @@ macro_rules! impl_discrete_trait {
             }
 
             fn from_index(&self, index: usize) -> Option<Self::ValueType> {
-                if let Ok(index) = Self::ValueType::try_from(index) {
-                    return Some(self.0 + index);
-                }
-                None
+                // Ensure index can be added to self.0 without overflowing
+                self.0.checked_add(index as Self::ValueType)
             }
         }
     };