Adds unicode support to Windows environment.
Adds unicode support to Windows environment.

Currently the Windows environment uses the *A ANSI Win32 API calls for file system operations. This precludes the ability of consuming applications to pass paths with unicode characters. A detailed discussion is in #755. This PR swtiches the *A ANSI calls to the *W wide unicode string calls along with the conversion methods from standard UTF-8 strings to the UTF-16 multi-byte strings requires by the Win32 *W API functions.

bitcoin-core/leveldb,bitcoin-core/leveldb,bitcoin-core/leveldb