Working with no-std

[IshitaTakeshi]

I found that kdtree-rs can be used for development of robot localization algorithms on edge devices, so I made this crate compatible with no-std.

The main modification was to replace std functions with the ones in the core crate. The thiserror crate seems only available on std, so it will be disabled on no-std.

I did this modification just for my use case. If this PR is not necessary for you, feel free to close without merge.

[mrhooray]

Thanks for the PR. I'm not familiar with no std setup - current state fails when testing with std feature

cargo test --features std
warning: no edition set: defaulting to the 2015 edition while the latest is 2021
   Compiling kdtree v0.7.0 (/Users/rui/Projects/kdtree-rs)
error[E0433]: failed to resolve: you might be missing crate `core`
 --> src/heap_element.rs:2:5
  |
2 | use core::cmp::Ordering;
  |     ^^^^
  |     |
  |     you might be missing crate `core`
  |     help: try using `std` instead of `core`: `std`

error[E0433]: failed to resolve: you might be missing crate `core`
  --> src/util.rs:25:9
   |
25 |     use core::f64::{INFINITY, NEG_INFINITY};
   |         ^^^^
   |         |
   |         you might be missing crate `core`
   |         help: try using `std` instead of `core`: `std`
...