Rewrite set_logger function to use std::sync::Once on std

I split implementations for 3 cases:
* When std is available, use standard locking primitive.
* When std is not available but there is atomics with pointer size, use spinlock
* If there is no atomic with pointer size, assume that there is no threads and use `Cell`.

I think, this separation makes easier to understand what is happening in each case.

src/__private_api.rs

@@ -2,8 +2,8 @@
 
 use self::sealed::KVs;
 use crate::{Level, Metadata, Record};
-use std::fmt::Arguments;
-pub use std::{file, format_args, line, module_path, stringify};
+use core::fmt::Arguments;
+pub use core::{file, format_args, line, module_path, stringify};
 
 #[cfg(feature = "kv_unstable")]
 pub type Value<'a> = dyn crate::kv::value::ToValue + 'a;