Make IO#read_char's default behaviour UTF-8-strict

[HertzDevil]

A lot of invalid UTF-8 byte sequences go through IO#read_char unnoticed on the default encoding settings where decoding support is provided by Crystal itself instead of Iconv. For example:

io = IO::Memory.new(Bytes[0x81, 0x00])
io.read_char # => '@'
io.pos       # => 2

io = IO::Memory.new(Bytes[0xED, 0xA0, 0x80]) # U+D800
io.read_char # => '�'
io.pos       # => 3

io = IO::Memory.new(Bytes[0xF7, 0xBF, 0xBF, 0xBF]) # U+1FFFFF
io.read_char.not_nil!.ord # => 2097151
io.pos                    # => 4

This PR adds the same checks as those in Char::Reader#decode_char_at which conform to UTF-8.

[straight-shoota]

Would it be possible to combine both methods with a little refactoring? They seem pretty similar, so it might be relatively easy to do.

Alternative, there should be comments referencing the respective other methods.

[straight-shoota]

Oh, I actually meant Char::Reader#decode_char_at and IO#read_char. This doesn't look too bad, though, either...