Rename `mrb_shared_string::len` to `mrb_shared_string::capa`

Also use `str_init_shared` for `orig` in `str_make_shared()`

Prohibit changes to iseq in principle

Because this field is used as capacity of string buffer.

Refactor set/unset string type flags

Introduce `RSTR_SET_TYPE_FLAG` macro to set the specified string type flag and
clear the others.

Fix `String#rindex` with invalid UTF-8 string

Previously `String#rindex` returned the wrong index when given an
invalid UTF-8 string.

```terminal
% ruby26 -e 'str = "\xf0☀\xf1☁\xf2☂\xf3☃\xf0☀\xf1☁\xf2☂\xf3☃"; p str.rindex("☁")'
11
% ./mruby-head -e 'str = "\xf0☀\xf1☁\xf2☂\xf3☃\xf0☀\xf1☁\xf2☂\xf3☃"; p str.rindex("☁")'
nil
% ./mruby-patched -e 'str = "\xf0☀\xf1☁\xf2☂\xf3☃\xf0☀\xf1☁\xf2☂\xf3☃"; p str.rindex("☁")'
11
```

Merge pull request #4633 from shuujii/shared-fshared-string-is-not-required-when-sharing-nofree-string

SHARED/FSHARED string is not required when sharing NOFREE string

I think the string buffer of NOFREE string always exists and does not need
to be released, so it can be shared as another NOFREE string.

Also changed the `mrb_shared_string` field order so that eliminate padding if
`int` and `mrb_int` sizes are less than pointer size.

But this changes requires `OP_ARYCAT` and `OP_ARYPUSH` to accept `nil`
as their first operand. Alternative VMs (e.g. `mruby/c`) that understand
mruby bytecode need to be updated.

Some `undef' functions may be called before initialization, thus causes
infinite error recursion.

Remove unneeded `#include` in `src/string.c`

Merge pull request #4631 from shuujii/extract-initialization-code-of-shared-and-fshared-string-to-function

Extract initialization code of shared and fshared string to function

Avoid calling `initialize` via `mrb_funcall`, which cause `cross C
boundary` error from Fibers started in the method.

* `mrb_vm_define_class`
* `mrb_vm_define_module`

Only functions called from user code requires `MRB_API`.

Extract `struct RString` initialization code to function

`mrb_str_pool` can embed one more byte; ref #4626

`mrb_str_modify_keep_ascii` can embed one more byte

This reverts commit 3dc8d9d7.