standard lazy types

[matklad]

Add support for lazy initialized values to standard library, effectively superseding the popular lazy_static crate.

use std::sync::Lazy;

// `BACKTRACE` implements `Deref<Target = Option<String>>` 
// and is initialized on the first access
static BACKTRACE: Lazy<Option<String>> = Lazy::new(|| {
    std::env::var("RUST_BACKTRACE").ok()
});

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