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
• OpenTelemetry with PythonCall can yield LLVM ERROR: Cannot emit physreg copy instruction #80