feat(TinyVec): add into_vec() & into_boxed_slice()

[2moe]

1. rustfmt.toml: replace deprecated fn_args_layout with fn_args_layout
   see also: Rename fn_args_layout config option rust-lang/rustfmt#4149

2. benches/smallvec.rs: After running cargo +nightly fmt, the format was automatically changed.

3. src/tinyvec.rs:

• add impl<A: Array> Into<Vec<A::Item>> for TinyVec<A>
• add into_vec() & into_boxed_slice()

---

into_vec()

The specific implementation is in the into() method of impl Into<Vec<A::Item>>.

into_boxed_slice()

Converts a TinyVec<[T; N]> into a Box<[T]>.

• For TinyVec::Heap(Vec<T>), it takes the Vec<T> and converts it into
  a Box<[T]> without heap reallocation.
• For TinyVec::Inline(inner_data), it first converts the inner_data to
  Vec<T>, then into a Box<[T]>. Requiring only a single heap
  allocation.

Example

use core::mem::size_of_val as mem_size_of;
use tinyvec::TinyVec;

// Initialize TinyVec with 256 elements (exceeding inline capacity)
let v: TinyVec<[_; 128]> = (0u8..=255).collect();

assert!(v.is_heap());
assert_eq!(mem_size_of(&v), 136); // mem size of TinyVec<[u8; N]>: N+8
assert_eq!(v.len(), 256);

let boxed = v.into_boxed_slice();
assert_eq!(mem_size_of(&boxed), 16); // mem size of Box<[u8]>: 16 bytes (fat pointer)
assert_eq!(boxed.len(), 256);

Closes #205

[Lokathor]

Looks good, just one little typo in the doc comments

[Lokathor]

I think this should say Box<[T]>

[Lokathor]

oops, might want to pick smaller array sizes for the doctest example

[2moe]

oops, might want to pick smaller array sizes for the doctest example

Now, TinyVec<[_; 102400]> -> TinyVec<[_; 128]>

[2moe]

When I changed the doc comment: Box<T> -> Box<[T]>, I forgot to run cargo fmt, which caused incorrect formatting.

I re-ran cargo fmt in smaller array sizes.