Fix build failures with `enable_debug` and `enable_cxx_abi`

Fix the Authors link in the README

chore: fix spelling

Under C++, there is no implicit conversion from `int` to `enum`, which caused a compilation error.

Merge branch 'define-MRB_NO_GEMS-if-disable_libmruby-is-specified' of https://github.com/shuujii/mruby into shuujii-define-MRB_NO_GEMS-if-disable_libmruby-is-specified

Fix build with `MRB_USE_ALL_SYMBOLS`

- (old) `undefined method 'foo'`
- (new) `no superclass method 'foo'`

Add support for partial backtraces

When argument information is not available. So it should not happen for
`yield` (error). In contrast, the error from `super` should be handled
in run time (ignored).

mrbc: Dump debug info with -g option

Adds debug source information (line/file) when mrbc uses -g.
This commit results in usable backtraces for all gems when build_config
is setup with enable_debug.

When debug information is omitted within ireps, show that a stack
frame existed rather than silently hiding it.

Revert "Fix `SIGSEGV` with mruby-method + mruby-catch"

Fix `SIGSEGV` with mruby-method + mruby-catch

- `mrb_num_div_int(mrb,x,y)` -> `mrb_div_int(mrb,x,y)`
- `mrb_num_div_flo(mrb,x,y)` -> `mrb_div_flo(x,y)`

They are internal function not supposed to be used outside of the core.

Previously, the following code would cause a `SIGSEGV`.

```ruby
mm = method(:throw)
define_method(:throw, ->(*args) { mm.call(*args) })
catch { |tag| throw tag }
```

I think the reason is in the `mrb_yield_with_class()` function:
- Even if a C function is called, `CI_ACC_SKIP` is used
- `cipop()` is not done if globally jumping from a C function

Tests for (`Float` or `Integer`) `op` `Complex`. Also added test
dependency to `mruby-rational` since `int_div` definition relies on
`Rational` when `MRB_USE_RATIONAL` is defined.

Tests for (`Float` or `Integer`) `op` `Rational`.

It removes non-static function, so that strictly saying, it's
an incompatible change. But the function was added recently and I am
sure no one uses it yet.