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.

`mrb_gc_destroy` may call other mruby API functions from the finalizers.
So call that function first.

The condition to make an embedded string was incorrect. Because there were
several similar codes, extracted into `RSTR_EMBEDDABLE_P` macro.

Remove monkey patches for Ruby 1.9 or earlier

`Enumerable#reject`, etc. should return `Enumerable` without block

Avoid premature overflow in Complex#abs and Complex#/

Math.hypot avoids premature overflow and underflow

Improve MiniRake rule enhancement for Rake compatibility

Support pathmap notation, recursive dependencies search, and so on.

Contrary to the name, `mrb_to_str` just checks type, no conversion.

`mrb_string_value_cstr` and `mrb_string_value_len`: obsolete
`mrb_string_cstr`: new function to retrieve NULL terminated C string
`RSTRING_CSTR`: wrapper macro of `mrb_string_cstr`