implement on `write` to a nested structure

[yellowbean]

Support function:

• set_val(path, val) -> replace the value inside the data structure where the path points to with val
• transform(path, func) -> apply the func to the value where path points to

The path should be compatible with the query function.

Research:

• Using https://github.com/MagicStack/immutables
• Probably using the recursive again , while it may encounter a performance hit .