There's no need for a `string` function since one already
exists for Nodes.
Now the root node is always stored on the document. This
means callers of `ts_document_root_node` don't need to release
its return value.
Generated parsers no longer export a parser constructor function.
They now export an opaque Language object which can be set on
Documents directly. This way, the logic for constructing parsers
lives entirely in the runtime. The Languages are just structs which
have no load-time dependency on the runtime
