New lint: vec.extend(&vec![elem; len]) is slow, vec.resize() should be used instead

There are some very slow ways of extending a vector with `len` new elements:
```rust
vec.extend(&vec![elem; len]);
vec.extend_from_slice(&vec![elem; len]);
```
Instead it should be written like this:
```rust
vec.resize(vec.len() + len, elem);
```
This sounds like code that nobody would ever write, but it actually has real-world precedent: https://github.com/Frommi/miniz_oxide/pull/55