Use thin-dst in GreenNode

[CAD97]

On top of #41. (Theoretically disjoint, but no size benefit without.)

Uses thin-dst to store GreenNode's kind, text_len, and children all inline in a single allocation, behind a thin pointer.

GreenElement can theoretically be further squished from 2×usize to 1×usize by using alignment bits to tag whether the pointer is a node or a token. That will be proposed in a follow-up PR.

r? @matklad

Size comparison

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

[matklad]

bors r+

[bors[bot]]

Build succeeded

• continuous-integration/travis-ci/push