collections: Put import behind cfg

[febo]

Problem

There is a warning for unused import when building the collections.

warning: unused import: `from_utf8`
  --> src/str.rs:25:15
   |
25 |         str::{from_utf8, from_utf8_unchecked},
   |               ^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` (part of `#[warn(unused)]`) on by default

Solution

Put the import behind the features that use it.

[febo]

CI runs with --all-features so this is why it was not picked up.