Identical code folding

Since Rust can't do polymorphization properly yet, using generics generates a lot of duplicated functions because of monomorphization. These functions take space in the binary, even though they have completely the same instructions.

Some linkers (`gold`, `lld`) can deduplicate these identical functions using **Identical Code Folding**, and thus reduce binary size (and potentially also improve usage of the i-cache).

You can specify this linker option using a [linker flag](https://tetzank.github.io/posts/identical-code-folding/), for example like this:
```bash
$ RUSTFLAGS="-Clink-args=-fuse-ld=lld -Clink-args=-Wl,--icf=all" cargo build
```

I measured the binary size change for the following program:
```rust
fn foo() {
let mut a: Vec = vec![1, 2, 3];
a.pop();

let mut b: Vec = vec![1, 2, 3];
b.pop();

let mut c: Vec = vec![1, 2, 3];
c.pop();
}

fn main() {
foo();
}
```

Here are binary sizes are after running `strip` on them:

| Linker | Mode | Binary size (B) | ICF (Identical Code Folding) |
|--------|---------|-----------------|------------------------------|
| gold | debug | 342696 | No |
| gold | debug | 330408 | Yes |
| gold | release | 322216 | No |
| gold | release | 318120 | Yes |
| lld | debug | 330968 | No |
| lld | debug | 321840 | Yes |
| lld | release | 310616 | No |
| lld | release | 306848 | Yes |