feat: Add splice

[Marwes]

To mirror https://doc.rust-lang.org/alloc/vec/struct.Vec.html#method.splice .

The implementation is a straight copy of Vecs implementation, only
altered to call len()/reserve() instead of accessing the internal
len and buf field respectively as those do not exist in SmallVec.
An extra type parameter were also required for Drain.

[emilio]

I haven't dug through all the code yet, but this needs a bunch of tests for the relevant edge cases, at the very least.

[Marwes]

Copied over the tests for Vec::splice, those don't test the additional edge conditions that SmallVec has but its a start.

[emilio]

Seems there's something wrong as tests are failing. The error looks legit:

error: Miri evaluation error: Memory access failed: pointer must be in-bounds at offset 24, but is outside bounds of allocation 25122 which has size 20
    --> /home/travis/.rustup/toolchains/nightly-2020-01-08-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/intrinsics.rs:1586:5
     |
1586 |     copy(src, dst, count)
     |     ^^^^^^^^^^^^^^^^^^^^^ Miri evaluation error: Memory access failed: pointer must be in-bounds at offset 24, but is outside bounds of allocation 25122 which has size 20
     |
     = note: inside call to `std::intrinsics::copy::<i32>` at /home/travis/build/servo/rust-smallvec/lib.rs:360:9
     = note: inside call to `smallvec::Drain::<[i32; 1]>::move_tail` at /home/travis/build/servo/rust-smallvec/lib.rs:302:17
     = note: inside call to `<smallvec::Splice<std::iter::Cloned<std::slice::Iter<i32>>, [i32; 1]> as std::ops::Drop>::drop` at /home/travis/.rustup/toolchains/nightly-2020-01-08-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/ptr/mod.rs:184:1
note: inside call to `std::ptr::real_drop_in_place::<smallvec::Splice<std::iter::Cloned<std::slice::Iter<i32>>, [i32; 1]>> - shim(Some(smallvec::Splice<std::iter::Cloned<std::slice::Iter<i32>>, [i32; 1]>))` at tests/splice.rs:7:38
    --> tests/splice.rs:7:38
     |
7    |     v.splice(2..4, a.iter().cloned());
     |                                      ^
note: inside call to `test_splice` at tests/splice.rs:4:1
    --> tests/splice.rs:4:1
     |
4    | / fn test_splice() {
5    | |     let mut v: SmallVec<[_; 1]> = smallvec![1, 2, 3, 4, 5];
6    | |     let a = [10, 11, 12];
7    | |     v.splice(2..4, a.iter().cloned());
...    |
10   | |     assert_eq!(&v[..], &[1, 20, 11, 12, 5]);
11   | | }
     | |_^