Fix segv on str_buf_cat

Modify from nofree to embed string

Also more size checks added.

The issue was reported by https://hackerone.com/ston3

Check array max size

Fix #3354

Fix segv when primitive value

Fix #3352

Check overflow string length

Do nothing when empty string

Fix #3360

Fix #3361

This issue was reported by https://hackerone.com/mg36

Added example for cross-compiling on Renesas RX630

Reported by https://hackerone.com/haquaman.

Reported by Denis Kasak https://hackerone.com/dkasak

Fix crash when exponent is -2147483648

Check type before calling mrb_range_ptr.

Reported from @clayton-shopify.

Also msym size changed to 512 from 256.