Include time.h for ios

C++ is stricter in implicit type casting.

Differences from the PR #4940:
* Use simple search for short strings only.
* "short" means `m+n` is shorter than `MRB_QS_SHORT_STRING_LENGTH`.
* The current default value for `MRB_QS_SHORT_STRING_LENGTH` is 2048.

Fix builds for old mingw in mruby-socket; ref #4914

that includes `float.h`. It allows definitions from native headers.

My cat stepped on the keyboard at the last moment before the commit.

Support bit flags for `IO.open`

In top-level, `mid` is `NULL`. We used to ignore 'mid` update for `NULL`.

Delete duplicated `Array#delete_if`.

This bug was introduced in 694089fa to address #4832

Issue 19902: mruby:mruby_fuzzer: Stack-buffer-overflow in mrb_str_len_to_dbl

This is reported by oss-fuzz:
Issue 19886: mruby:mruby_fuzzer: Potential-null-reference in setup_numparams

Merge branch 'file-size-truncate' of https://github.com/dearblue/mruby into dearblue-file-size-truncate

The basic idea of this change is from @dearblue.

Note: the arguments of `mrb_str_pool()` have changed, but the function
is provided for internal use (No `MRB_API`). So basically you don't have
to worry about the change.

Fix ainfo with keyword arguments; fix #4921