Support immutable data structure

[gcao]

Add immutable variant for most of the ValueKinds

Register immutable variants

Infer / validate immutability

Support immutable data on class level - how?

Convert between mutable and immutable data

var vs const

Use cases for immutability

https://en.wikipedia.org/wiki/Immutable_object
https://medium.com/tribalscale/understanding-immutability-fdd627b66e58