Make `KVValue` a readonly struct

I'm not sure how to accomplish this though because it's extended by `KVObjectValue`.

In the VRF library we've done it trivially: https://github.com/ValveResourceFormat/ValveResourceFormat/commit/7dd63c0310b0c2154c6a4f1dff6124f5b2b514e4

@yaakov-h got ideas?

Related: #30