Convert Into<Box<[T]>> for Vec<T> into From<Vec<T>> for Box<[T]>

[ollie27]

As the collections crate has been merged into alloc in #42648 this impl is now possible. This is the final part of #42129 missing from #42227.

[rust-highfive]

r? @sfackler

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[Mark-Simulacrum]

Travis failed:

[01:02:51] error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g. `MyStruct<T>`); only traits defined in the current crate can be implemented for a type parameter
[01:02:51] --> /checkout/src/liballoc/vec.rs:2128:1
[01:02:51] |
[01:02:51] 2128 | / impl<T> From<Vec<T>> for Box<[T]> {
[01:02:51] 2129 | | fn from(v: Vec<T>) -> Box<[T]> {
[01:02:51] 2130 | | v.into_boxed_slice()
[01:02:51] 2131 | | }
[01:02:51] 2132 | | }
[01:02:51] | |_^

[ollie27]

Well I didn't expect that. I've added #[cfg(not(test))] like impl<T> From<Box<[T]>> for Vec<T> has which will hopefully fix it.

[tomprince]

@bors try
Triggering a build to run this through cargobomb.

[bors]

⌛ Trying commit 222a328 with merge 5311126...

[bors]

☀️ Test successful - status-travis
State: approved= try=True

[sfackler]

@bors r+

[bors]

📌 Commit 222a328 has been approved by sfackler

[frewsxcv]

@bors rollup