Unlike method definition, constant reference should start from
`MRB_PROC_TARGET_CLASS(ci->proc)`, not `ci->target_class`.
In addition, `MRB_PROC_TARGET_CLASS(ci->proc)` is always set.

The destination `proc` may be an orphan.

The issue #3837 is a regression of #3819. I was mistakenly removed
a important code to mark fibers from `env`.

The change is suggested by `ukrainskiysergey` in #3836.
I deleted the other line for clarity. I also updated comment lines.

This is a resurrection of d0a7e01d, which is accidentally
removed by 93f5f225; Fix #3715

This is a resurrection of 75c374ca, which is accidentally removed by
93f5f225; Fix #3507 #3512 #3518 #3521

Both `uvenv` function and `env` generation in `create_proc_from_string`
function have bugs to handling enclosed environment objects.

Instead of `irep` links, we added a `upper` link to `struct RProc`.
To make a space for the `upper` link, we moved `target_class` reference.
If a `Proc` does not have `env`, `target_class` is saved in an `union`
shared with `env` (if a `Proc` has env, you can tell it by `MRB_PROC_ENV_P()).
Otherwise `target_class` is referenced from `env->c`. We removed links
in `env` as well.

This change removes 2 members from `mrb_irep` struct, thus saving 2
words per method/proc/block. This also fixes potential memory leaks
due to the circular references caused by a link from `mrb_irep`.

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