migrate from thin-dst to slice-dst and erasable

[CAD97]

thin-dst is deprecated in favor of using slice-dst and erasable, which provide a more principled and build-your-abstraction API.

This just ports the existing structure to use erasable::Thin<Arc<slice_dst::SliceWithHeader<_, _>>> instead of thin_dst::ThinArc<_, _>.

[CAD97]

bors: r+

This is a minor refactoring and a pre-requisite of exposing a GC for the cache.

[CAD97]

CI passed, but the .org Travis seems to have not been able to report that back. https://travis-ci.org/github/rust-analyzer/rowan/builds/699851971

cc @matklad we'll probably need to either migrate to .com Travis or GHA.

Merging manually. bors: cancel

[CAD97]

bors: r-

[bors[bot]]

Canceled.