Fix that sometimes parallel build of test code fails; fix #5284

The cause is that `mrbgem.rake` of` mruby-test` gem is loaded when test
code is requested to be built, but when `mrbgem.rake` is loaded,
`MRuby::Gem.current` is updated, which is not thread safe.

Address this by not loading `mrbgem.rake` in parallel.

To make inline symbols packed in 30 bits.

Remove functions for unimplemented methods

Integrate the argument parsing part of `IO.popen`

- Use `mrb_notimplement_m()` instead.
- Hide the unused `option_to_fd()` when `TARGET_OS_IPHONE` is enabled.

Add missing cast in `ea_next_capa_for`

Create a `mruby-config` that can be run on the host by cross-building

The output directory will be `host-bin`, so it will be output as `<build-dir>/host-bin/mruby-config`.

It's still not available for target devices.

Merge pull request #5281 from shuujii/remove-the-definition-of-mrb_uint-in-numeric.h-that-is-no-longer-needed

Remove the definition of `mrb_uint` in `numeric.h` that is no longer needed

Remove unneeded check to `mrbtest` in `tasks/presym.rake`

With the change in #5267, `build.products` no longer contains `mrbtest` when
`tasks/presym.rake` is loaded.

Remove unneeded `mruby-test` gem in `build_config/no-float.rb` [ci skip]

Use `enable_test` to enable the test.

Fix annotations [ci skip]

`1L` on Windows means `32 bit int`.

Fix build error in cross-build with presym

Replace `tempirep` with `RProc`

Fix the condition to remove `mrbtest` when `rake clean`

Previously I used the `RData` object to avoid a memory leak in `mrb_irep` if `src/load.c` failed.
ref: https://github.com/mruby/mruby/pull/4250
commit: f1523d24

Considering that the `RProc` object will be created in the subsequent process, it is preferable to create the `RProc` object from the beginning.
Along with this, the inside of `read_irep()` is replaced with the processing centered on the `RProc` object.

The global function that returns the `mrb_irep` pointer is still provided for compatibility.

This reverts commit 37d795de.
This reapplies e20d652f (#4202), which is reverted, but its issue was solved
by 1d8456fe.

This reverts commit a0c1e075.

This is because the `mrb_callinfo::pc` has been reorganized, resulting in over-correction.

This enhances self-containment.

- Changed the `mrb_callinfo::pc` field to point to itself.
  Previously it indicated the return destination of the previous call level.
  `mrb_callinfo::pc` will now hold the address to its own `proc->body.irep->iseq`.
- Removed `mrb_callinfo::err` field.
  This is because `mrb_callinfo::pc - 1` is semantically the same as the previous `err`.
- The `pc0` and `pc_save` variables in `mrb_vm_exec()` are no longer needed and have been deleted.
- It removes the argument because `cipush()` doesn't need to save the previous `pc`.

This enhances self-containment.

Previously `mrb_context::stack` had the current call level stack, but now it owns it.
The `mrb_context::stack` field, which is no longer needed, will be removed.

If there is `env`, `env->c` means `target_class`.

This is because it is enough to express the range up to (-1..255) or (-3..255).

Fix the following issues.

* https://github.com/mruby/mruby/pull/5220#issuecomment-757204258
* https://github.com/mruby/mruby/pull/5220#issuecomment-757205572

The first byte of UTF-8 character should not be `80..c1`.