Add ast support module, extracted from rust-analyzer

[mullr]

I used this code from rust-analyzer in order to be able to build out my own AST structures. The only change I made was to make it generic over language.

[mullr]

Is there any desire to have this code in the main rowan repo? If not, I can make a separate utility crate. Just don't want to maintain an unnecessary fork.

[Veykril]

Ah, can you bump the rowan version to 0.15.4? Then merging will auto-publish the new minor version

[mullr]

Ah, can you bump the rowan version to 0.15.4?

Done

[Veykril]

bors r+

[bors[bot]]

Build succeeded:

• Rust