Rollup merge of #103718 - matklad:infer-lazy, r=dtolnay

Rollup merge of #103718 - matklad:infer-lazy, r=dtolnay

More inference-friendly API for lazy

The signature for new was

```
fn new<F>(f: F) -> Lazy<T, F>
```

Notably, with `F` unconstrained, `T` can be literally anything, and just `let _ = Lazy::new(|| 92)` would not typecheck.

This historiacally was a necessity -- `new` is a `const` function, it couldn't have any bounds. Today though, we can move `new` under the `F: FnOnce() -> T` bound, which gives the compiler enough data to infer the type of T from closure.