fix: build errors for Node

[mayant15]

There are some calls in src/node.zig that don't compile:

1. currentFieldId() and currentNode() don't exist on TreeCursor but fieldId() and node() do.
2. Some calls to toOwnedSlice() do not have an allocator.

The first one seems to be a deliberate change (7f531d3), so maybe you'd prefer I fix method names in TreeCursor instead? Let me know! ^-^