Improve `std::collections`

[RustPowers]

1. Move std::vec to the module std::collections. Why? He's taken out of context. Directly hardly anyone uses std::vec module. In addition, the Vec object is re-exported with std::preluge, so the breaking is small.
2. Remove re-exports in std::collections and add std::collections::preluge. (Similar to the module std::io::preluge)

[apasel422]

1. Please see Put Vec in with the rest of the collections #34944.
2. Removing the re-exports would be a breaking change.

[brson]

Thanks for the report but these changes have been discussed before and are not going to be made.