Turn `immutable` structs with `Ref` fields into `mutable`

[findmyway]

The motivation was to avoid modifying some fields. Given that since [email protected], we can mark immutable fields in a mutable struct with const, better to turn those into mutable not.

Ref:

• https://docs.julialang.org/en/v1/manual/types/#Mutable-Composite-Types
• #80