Add default exception handler for win32

[TechnicalFowl]

This adds a default UnhandledExceptionFilter to the stdlib for win32. This will be called when an exception (such as a seg fault) reaches the top-level unhandled exception filter and the application is not being debugged. Currently the application will just silently die in these cases necessitating breaking out the debugger but this will report a convenient backtrace.

It also registers a panic function to allow it to track if a panic has occurred to prevent it double reporting these when the panic function calls $$trap().

Example:

import std::io;

fn void main()
{
    io::printfn("Foo");
    uint* p = (uint*) 0x13370;
    *p = 0;
    io::printfn("Bar");
}

Previous output:

Foo

New output:

Foo

ERROR: 'Access Violation at address 0x7ff6fe370f8a'
  in main (C:\Data\test\src\main.c3:52) [C:\Data\test\build\test.exe]
  in _$main (C:\Data\test\src\main.c3:47) [C:\Data\test\build\test.exe]
  in invoke_main (D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:78) [C:\Data\test\build\test.exe]
  in __scrt_common_main_seh (D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288) [C:\Data\test\build\test.exe]
  in __scrt_common_main (D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:330) [C:\Data\test\build\test.exe]
  in mainCRTStartup (D:\a\_work\1\s\src\vctools\crt\vcstartup\src\startup\exe_main.cpp:16) [C:\Data\test\build\test.exe]
  in BaseThreadInitThunk (source unavailable) [C:\WINDOWS\System32\KERNEL32.DLL]
  in RtlUserThreadStart (source unavailable) [C:\WINDOWS\SYSTEM32\ntdll.dll]

[lerno]

Thank you!