libcollections: remove `init_to_vec`

The `init_to_vec` function in `collections::bitv` was exposed as an
inherent method, but appears to just be a helper function for the
`to_vec` method. This patch inlines the definition, removing
`init_to_vec` from the public API.

[breaking-change]
auto merge of #14295 : aturon/rust/hide-init_to_vec, r=alexcrichton

The `init_to_vec` function in `collections::bitv` was exposed as an
inherent method, but appears to just be a helper function for the
`to_vec` method. This patch inlines the definition, removing
`init_to_vec` from the public API.

[breaking-change]