Add `utf16_units`

This deprecates `.to_utf16`. `x.to_utf16()` should be replaced by either
`x.utf16_units().collect::<Vec<u16>>()` (the type annotation may be optional), or
just `x.utf16_units()` directly, if it can be used in an iterator context.

Closes #14358

[breaking-change]
auto merge of #14613 : schmee/rust/utf16-iterator, r=huonw

Closes #14358.

~~The tests are not yet moved to `utf16_iter`, so this probably won't compile. I'm submitting this PR anyway so it can be reviewed and since it was mentioned in #14611.~~ EDIT: Tests now use `utf16_iter`.

This deprecates `.to_utf16`. `x.to_utf16()` should be replaced by either `x.utf16_iter().collect::<Vec<u16>>()` (the type annotation may be optional), or just `x.utf16_iter()` directly, if it can be used in an iterator context.

[breaking-change]

cc @huonw
Add os::join_paths, make setenv non-utf8 capable

This commit changes `os` in three ways:

* It adds a `join_paths` function that is the converse to `split_paths`,
  easing manipulation of the `PATH` environment variable according to
  platform conventions.

* **Breaking change**: It changes `split_paths` to no longer drop empty paths, since they are
  meaningful to some shells (where they are synonymous with the current
  working directory).

* It changes `setenv` to take a `BytesContainer` rather than a `&str`
  value, since environment variables may have non-utf8 values on some
  platforms. Since `&str` is a `BytesContainer`, this is *not* a
  breaking change.

Along the way, it also refactors the `split_paths` function so that
`cfg` switches are applied internally (and the function header is given
only once). This fixes a bug: the doc comment had an example for only
one platform.

[breaking-change]