Make Int#chr reject surrogate halves

[HertzDevil]

Resolves #10440.

This completes my own review of the standard library's calls to Int#chr and #unsafe_chr. The result is:

• Int#chr raises if self is within 0xD800 - 0xDFFF.
• Calls to #unsafe_chr in the standard library are guaranteed to be safe, either by surrounding code (offending calls will be fixed in Disallow surrogate halves in escape sequences of string and character literals #10443, Make IO#read_char's default behaviour UTF-8-strict #10446, Fix JSON::Lexer's UTF-16 escape sequence parsing #10450), or because they come from a Bytes or Pointer(UInt8) (the more common case). Note that a String can still hold invalid codepoints but safe access from outside would produce the replacement character.
• Char's own methods will now assume self always represents a valid UTF-8 codepoint, so #each_byte no longer raises on invalid ones.

[beta-ziliani]

Thanks!