## Summary
This following is a behavior from CRuby 1.8.7 to 3.1.0.
```console
% ruby -ve "p ['a', 'b', 'c']*''"
ruby 1.8.7 (2013-12-22 patchlevel 375) [i686-darwin13.0.2]
"abc"
% ruby -ve "p ['a', 'b', 'c']*''"
ruby 3.1.0p0 (2021-12-25 revision fb4df44d16) [x86_64-darwin19]
"abc"
```
### Before (mruby 3.0.0)
mruby unexpectedly gives the TypeError.
```ruby
['a', 'b', 'c']*'' #=> String cannot be converted to Integer (TypeError)
```
### After
This PR makes mruby behave compatible with CRuby.
```ruby
['a', 'b', 'c']*'' #=> 'abc'
```
As far as I checked, the behavior is unspecified when `Array#*`'s argument
is not an instance of Integer class in X 3017 : 2013 (ISO/IEC 30170 : 2012).
## Additional Information
I noticed this difference by the following idiom when writing ASCII art code
using Ruby.
```ruby
%w(foo bar baz)*''
```
e.g. TRICK (https://github.com/tric)
We used to check them by `mrb_ensure_xxx_type()` functions, but type
errors there should not occur if there's no bug in code generations.
So we use assertion rather than dynamic type checks.
ref. #2485
The `SystemCallError#to_s` method also needed to be modified, but since it has no functional difference from the `Exception#to_s` method, it will be removed.
ref. commit 7f40b645d2
Currently, the build configurations `MRB_USE_COMPLEX` and `MRB_USE_RATIONAL` are not listed in the documentation.
In other words, they are hidden settings.
They are defined in `mrbgems/mruby-{complex,rational}/mrbgem.rake`.
So this patch assumes that it is safe to refer to these functions in core-gems directly from core functions.
However, applications that link with `libmruby_core.a` will have compatibility issues.
In fact, `mrbgems/mruby-bin-mrbc` links with `libmruby_core.a`, so I had to prepare a dummy function.
ref. #5613.
I mentioned in #5540 that there was no reentrant to the virtual machine, but in fact it was still a possibility at that point.
Also, the variable `ci` needs to be recalculated at the same time.
ref. #5613
I checked with Valgrind, and the methods that can cause use-after-free are `Array#rotate`, `Array#rotate!`, and `String#byteslice`.
Since `String#rindex` uses `RSTRING_LEN()` indirectly inside the function, no reference to the out-of-bounds range is generated.
Since `mrb_to_integer` and `mrb_to_float` does not convert the object
but checks types, they are named so by historical reason. We introduced
properly named functions.
This commit obsoletes the following functions:
* mrb_to_integer()
* mrb_to_int()
* mrb_to_float()
Use `mrb_ensure_int_type()` instead for the first 2 functions. Use
`mrb_ensure_float_type()` for the last.
Yukihiro "Matz" Matsumoto