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.
For the slight cost of counting code points when converting between
encodings and a teeny bit of memory, this commit adds a fast path for
all-happy utf-16 substrings and code point operations.
This seems to be a significant chunk of time spent in many regex
benchmarks.
