* Error and ErrorOr are not themelves constexpr, so a function returning
these types cannot be constexpr.
* The UDL was trying to call Utf16View::validate, which is not constexpr
itself. The compiler will actually already raise an error if a UTF-16
literal is invalid, so let's just avoid the call altogether.
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.
This function converts a single wide character into its multibyte
representation (UTF-8 in our case). It is called from libc++'s
`std::basic_ostream<wchar_t>::flush`, which gets called at program exit
from a global destructor in order to flush `std::wcout`.
This introduces the UnicodeUtils file, which contains helper functions
related to Unicode. This is in contrast to StringUtils, whose functions
are not directly related to Unicode and are, in theory,
encoding-agnostic.
