Use `RBasic` padding for embedded string on 64-bit CPU

On 64-bit CPU, there is padding in `RBasic`, so reorder the fields and use
it as buffer of embedded string. This change allows 4 more bytes to be
embedded on 64-bit CPU.

However, an incompatibility will occur if `RString::as::ary` is accessed
directly because `RString` structure has changed.

Move `mrb_str_pool` to `src/string.c` to use `str_init` family

Allow external definition of MRB_API macro

When building WebAssembly targets with clang (e.g. wasm32-unknown-unknown), all
symbols are hidden by default. To export a symbol, it must be marked with

    __attribute__((visibility(default)))

Rather than detecting this exotic target in mruby, allow MRB_API to be defined
externally.

Suppress warnings for `strncat()`

Simplify get arguments

Generate doxygen docs for mruby

Make symbolic link names unique for test

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.

`strncat()` also needs `'\0'`.

- `mrb_str_index_m()` and `mrb_str_rindex()`
  Make `mrb_get_args()` called only once from called twice.
- `mrb_str_byteslice()`
  Replace `goto` with `if ~ else`.

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.