The motivation is that the proposals are not experimental *code*, but
rather, they are documents that are meant to be read, and happen to have
some bits that you can copy over to the rust playground.

So in that respect, the texts are no different than any other doc you
may write, hence the move to proposals.

This change contributes an implementation of the [Intl.ListFormat
API][lfapi], as prescribed by [ECMA-402][ecma].

The ECMA-402 API surface is developed in lock-step with a `rust_icu`
backed implementation.

The approach is documented [here][appr].

  [lfapi]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/ListFormat
  [ecma]: https://www.ecma-international.org/publications/standards/Ecma-402.htm
  [appr]: https://raw.githubusercontent.com/google/rust_icu/master/experimental/ecma402_listformat/src/lib.rs

See issue google#125