Make meta verify objects are JSON serialisable before insertion.

[0Hughman0]

If you insert non JSON data into a meta dict, everything freaks out because whenever meta tries to write to disk, it can't.

We should verify something is serialisable prior to insertion.

[0Hughman0]

This should be done using pydantic.

I think it should be possible to enforce errors for incorrect types in meta, although I can see this becoming a bit of a mess.

[0Hughman0]

Type hints introduced in #53 should help a bit. Hints suggest values must be JSONType

Cassini/cassini/accessors.py

Line 27 in 7116ee0

JSONType = Union[dict, list, tuple, JSONPrimative]