Unicode Compiler Error Fix

[AMZN-Gene]

Using TCHAR string to fix compiler error when compiling with UNICODE
TCHAR automatically resolves to wchar or char depending if unicode is enabled or not.

Resolves #213

[spham-amzn]

Instead of possibly converting the filename to a char or wide character, why no use CreateFileA directly so it always uses char? CreateFile is really a macro that switches between CreateFileA and CreateFileW based on the unicode switch anyways

[syoyo]

Personally I don't like TCHAR approach.

Filename in USD is usually UTF8, so we can assume input string(in std::string) is UTF8 encoded, and it'd be better to use CreateFileW (WideChar version of API), by converting UTF8 string to widechar string(UTF16LE in Windows).

Please take a look at a code fragment in TinyEXR: https://github.com/syoyo/tinyexr/blob/5fcb4dcb6e3abf96214b67e5c54db1ceec6a455c/tinyexr.h#L6746

The same implementation is also used in OpenUSD: PixarAnimationStudios/OpenUSD#1580

[syoyo]

I've fixed the issue in another branch.

857a539

It always uses CreateFileW/Widechar(UTF16LE) interface.

It also provides std::wstring interface for MMapFile(Windows only)

I'll merge mmap_file branch after some tests.

[AMZN-Gene]

Beautiful! Sorry I was pulled away from this for a while, but @syoyo fixed it!