Box and cache GreenToken

[CAD97]

This uses an inner Arc to decrease the size of GreenToken. This uses the same caching logic as GreenNode through GreenNodeBuilder. Do note that this means that to get a decently memory-thoughtful tree, either the use of GreenNodeBuilder or a manual cache in the same style is required. We should probably consider eventually exposing some version of the Cache implemented here, such that bottom-up builders can use it and get the deduplication for free.

The main reason for shrinking GreenToken is the target of shrinking GreenElement, as each node carries a [GreenElement], and by reducing overhead per-element, we can considerably decrease overall memory usage. By boxing GreenToken we make room to shrink GreenNode and benefit from said shrinkage.

r? @matklad

Size comparison

Before: ``` GreenNode 24 GreenToken 32 GreenElement 40 SyntaxNode 8 SyntaxToken 16 SyntaxElement 24 ``` After: ``` GreenNode 24 GreenToken 8 GreenElement 32 SyntaxNode 8 SyntaxToken 16 SyntaxElement 24 ```

[CAD97]

Next step towards a single-pointer GreenElement is writing up all the docs for thin-dst and publishing 1.0.0.

[matklad]

bors r+

[bors[bot]]

Build succeeded

• continuous-integration/travis-ci/push