Add Lazy::into_value

[matklad]

This API allows to move a T out of Lazy<T>. Note that it requires an owned access to a Lazy -- there's no way to reset a lazy to uninit state via &mut Lazy<T>, as the init function is gone. In other words, fn take(this: &mut Lazy<T>) is an impossible object.

[matklad]

bors r+

[matklad]

bors r-

[bors[bot]]

Canceled.

[matklad]

bors r+

[bors[bot]]

Build failed:

• Rust

[matklad]

bors r+

[bors[bot]]

Build succeeded:

• Rust