Panicking insert_many leaks memory

[RalfJung]

The test_insert_many_panic test leaks memory. Both of these allocations here to do get freed, presumably due to the panicking insert_many:

rust-smallvec/lib.rs

Lines 2096 to 2101 in 3957cd8

	PanicOnDoubleDrop {
	dropped: Box::new(false),
	},
	PanicOnDoubleDrop {
	dropped: Box::new(false),
	},

Found by Miri:

note: tracking was triggered
    --> /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/liballoc/alloc.rs:80:47
     |
80   |     __rust_alloc(layout.size(), layout.align())
     |                                               ^ created allocation with id 129830
     |
     = note: inside `alloc::alloc::alloc` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/liballoc/alloc.rs:80:47
     = note: inside `<alloc::alloc::Global as core::alloc::AllocRef>::alloc` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/liballoc/alloc.rs:174:49
     = note: inside `alloc::alloc::exchange_malloc` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/liballoc/alloc.rs:268:11
     = note: inside `alloc::boxed::Box::<bool>::new` at /home/r/.rustup/toolchains/miri/lib/rustlib/src/rust/src/liballoc/boxed.rs:174:9
note: inside `tests::test_insert_many_panic` at lib.rs:2100:26
    --> lib.rs:2100:26
     |
2100 |                 dropped: Box::new(false),
     |                          ^^^^^^^^^^^^^^^
note: inside closure at lib.rs:2072:5
    --> lib.rs:2072:5
     |
2072 | /     fn test_insert_many_panic() {
2073 | |         struct PanicOnDoubleDrop {
2074 | |             dropped: Box<bool>,
2075 | |         }
...    |
2107 | |         assert!(result.is_err());
2108 | |     }
     | |_____^

The following memory was leaked:
alloc129816 (Rust heap, size: 1, align: 1) {
    00                                              │ .
}
alloc129830 (Rust heap, size: 1, align: 1) {
    00                                              │ .
}

[mbrubeck]

This leak was intentional; see #96 and 26b2490 for details.

To fix this the leak without adding a double-free, the loop in insert_many will need to track which items are safe to drop, using a drop guard similar to the one used in from_elem and extend. (This might also have a performance impact, but I think fixing the leak is a higher priority than performance here, unless the performance impact is huge.)

[mbrubeck]

cc #186.