There were a couple of issues here, including the following computation
could actually overflow to NumericLimits<size_t>::max():
code_unit_offset -= it.length_in_code_units();
This was a mistake. Consider U+201C (LEFT DOUBLE QUOTATION MARK). This
code point is encoded as the bytes 0x1c 0x20 in UTF-16LE. Both of these
bytes are ASCII if interpreted as UTF-8. But the string itself is most
certainly not ASCII.
By definition, the web allows lonely surrogates by default. Let's have
our string APIs reflect this, so we don't have to pass an allow option
all over the place.
To prepare for an upcoming Utf16String, this migrates Utf16View to store
its data as a char16_t. Most function definitions are moved inline and
made constexpr.
This also adds a UDL to construct a Utf16View from a string literal:
auto string = u"hello"sv;
This let's us remove the NTTP Utf16View constructor, as we have found
that such constructors bloat binary size quite a bit.
Much of the web requires us to allow lonely surrogates in UTF-16 data.
The default behavior to disallow such code units has not been changed
here - that will be changed in an upcoming commit.
