Make all usages of `Obj` explicit

[sim642]

In 7edfff0 of #1326 an Obj-related bug was found which wasn't caught by the OCaml typechecker due to the Obj.
Although there's no simple way to avoid the use of Obj in MCP, we should always be explicit about using Obj, instead of open Obj and bringing all the wildly unsafe functions into scope. This would make it clearer and searchable, where the type system is being bypassed.

• Add Semgrep rule for open Obj.
• Remove all open Obj.
• Prefix all calls with Obj..