Forbid out of range Unicode code points.

Forbid out of range Unicode code points.

Unicode allows code points up to 0x10FFFF, but Perl allows much more.
However, code points above IV_MAX may not always work correctly, and
may even cause the interpreter to hang. Code points exceeding IV_MAX
have been deprecated since 5.24, and will be illegal in 5.28.

This commit removes many tests (without replacing them) as they were
testing behaviour of code points exceeding IV_MAX.