Fix string interoperability problem on using implicitly wchar_t is 16bit

[kekyo]

The entire IL2C relies on wchar_t being 16-bit.
I was completely oblivious to the fact that such an environment is possible...

• Explanation of the content of the issue: https://stackoverflow.com/questions/35982310/c-sharp-mono-passing-stringbuilder-to-fill-with-unicode-content-in-c?rq=1
• Test log:

  • IL2C/tests/IL2C.Core.Test.RuntimeSystems/Interoperability/Interoperability.cs

    Line 89 in d5f3bd5

    private static extern IntPtr wcslen(string message);

  • https://github.com/kekyo/IL2C/runs/6841226615?check_suite_focus=true#step:15:51

  Failed PInvokeWcsLen("ABC") [38 ms]
  Error Message:
     Expected: 3
  But was:  2
  Stack Trace:
     at NUnit.Framework.Internal.Commands.TestMethodCommand.Execute(TestExecutionContext context)
   at NUnit.Framework.Internal.Commands.BeforeAndAfterTestCommand.<>c__DisplayClass1_0.<Execute>b__0()
   at NUnit.Framework.Internal.Commands.DelegatingTestCommand.RunTestMethodInThreadAbortSafeZone(TestExecutionContext context, Action action)
  

Related #100

[kekyo]

Perhaps it would be better to introduce a type like char16_t so that it is always 16bit. However, because of the string literal problem, it cannot be said that simply defining a type and replacing it is sufficient, and it would be a bit involved in the discussion of whether to treat internally System.String as UTF8 or not.

Past related:

• Support System.Char #23 (comment)
• Support P/Invoke return type as string. #7

[NCLnclNCL]

Hello