`FrozenError` is a subclass of `RuntimeError` which used to be
raised.  [Ruby2.5]

Otherwise `super` may call a wrong method.

Now the method tables (in classes/modules and caches) keeps C function
pointers without wrapping in `struct RProc` objects. For the sake of
portability, `mrb_method_t` is represented by the struct and union, but
if the most significant bit of the pointer is not used by the platform,
`mrb_method_t` should be packed in `uintptr_t` to reduce memory usage.

`MRB_METHOD_TABLE_INLINE` is turned on by default for linux.

Fixes #3825

fix: mrbgems\mruby-kernel-ext\src\kernel.c(114): warning C4244: 'function': conversion from 'mrb_int' to 'int', possible loss of data

To enable method cache, define `MRB_METHOD_CACHE` or
`MRB_METHOD_CACHE_SIZE`. The cache size must be power of 2.
The default cache size is 128.

The measurement:
I measured simple benchmarks found in benchmark/ directory. With
method cache enabled, we gained 6-8% performance improvement, with
97-99% cache hit rate.

This only effects VC.

It will no longer shrink arena region. Instead `vm.c` uses
a static function `mrb_gc_arena_shrink()` to shrink.

This is a workaround for mingw-w64 (5.3.1) bug.

On my machine, gdb reverse-next does not work with memcpy and memset.
Add naive implementation of those functions to override.

This reverts commit eb5a606f and
079f310f.
The rescue stack works differently from ensure stack, so the change
caused #3686 and #3688. It might take long to solve the problems,
so that I would revert the changes for now. Fix #3688

Instead of preserving a backtrace in `mrb_state`, `mrb_exc_set`
keeps packed backtrace in an exception object. `#backtrace` unpacks
it to an array of strings.

A new function `mrb_yield_cont()` is provided. You have to call it
at the end of a C defined method, e.g. `return mrb_yield_cont()`.

This change reduces the recursion level, but does not solve the stack
overflow issue entirely.

close #3384
This issue was reported by https://hackerone.com/brakhane

Signed-off-by: Daniel Bovensiepen <daniel@bovensiepen.net>

So that other files can immediately use it.