add vec.capacity() to [slow_vec_initialization] detection

[DevAccentor]

fix #8800

for example

let mut vec1 = Vec::with_capacity(len);
vec1.resize(vec1.capacity(), 0);

let mut vec2 = Vec::with_capacity(len);
vec2.extend(repeat(0).take(vec2.capacity()));

will trigger the lint

---

changelog: add vec.capacity() to [slow_vec_initialization] detection

[rust-highfive]

r? @Manishearth

(rust-highfive has picked a reviewer for you, use r? to override)

[Manishearth]

@bors r+

thanks!

[bors]

📌 Commit 5a70d88 has been approved by Manishearth

[bors]

⌛ Testing commit 5a70d88 with merge f718984...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth
Pushing f718984 to master...