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.
Incorrect behavior of CreateProcess:
CreateProcess("a.exe", "b c", ...) ->
a.exe::main::argv == ["b", "c"] (wrong)
CreateProcess(0, "a.exe b c", ...) ->
a.exe::main::argv == ["a.exe", "b", "c"] (right)
https://learn.microsoft.com/en-us/cpp/cpp/main-function-command-line-args
"If you use both the first and second arguments (lpApplicationName and
lpCommandLine), argv[0] may not be the executable name."
This means first argument of CreateProcess should never be used.
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Searching for executable in path is suppressed now by prepending "./"
Additional bonus of the new code: .exe extension does not need to be
specified.
Windows doesn't have a concept of zombie children, hence:
* `disown` not needed
* we need a process handle because otherwise if the process have ended
by the time `wait_for_termination` is called
its pid may be reassigned to other process
