Backport bugfixes to release/1.8 for release 1.8.2

[straight-shoota]

This branch backports the following bugfixes to the release branch https://github.com/crystal-lang/crystal/tree/release/1.8:

• Fixup for JSON::Serializable on certain recursively defined types #13430
• Always use 0 for offset of StaticArray's @buffer #13319
• Fix String#scan with empty Regex match at multibyte char #13387
• Fix Log::Metadata#dup crash with 2+ entries #13369
• Refactor implementation of Iterator::ChainIterator #13412
• Check subject UTF8 validity just once for String#gsub, #scan, #split #13406

I have moved all of them to the 1.8.2 milestone.