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

[Shnatsel]

There are some very slow ways of extending a vector with len new elements:

vec.extend(&vec![elem; len]);
vec.extend_from_slice(&vec![elem; len]);

Instead it should be written like this:

vec.resize(vec.len() + len, elem);

This sounds like code that nobody would ever write, but it actually has real-world precedent: Frommi/miniz_oxide#55

[flip1995]

This is kind of similar to #4078, which is currently being implemented in #4647.