Implement checked::length_of

[the-moisrex]

Memory dump:

After blowup:
   ed 94 81 ed   85 b2 ed 0a   00 ed 0a 00   ed 0a 00 00   │ ················ │ cur len: 3, new len: 1
                        ^ ^    ^
                       0,1 \   3
                            2

Seems like required_length_of is not safe and it checks only "ed" which is the length of 3, but
it does not check the next code unit and what not.

[the-moisrex]

Done; even though I don't like the implementation of the UTF-8 version, but I like what I did with the UTF-16 version.