Use CreateProcessW instead of CreateProcessA

Changed libnative to use CreateProcessW instead of CreateProcessA.  In
addition, the lpEnvironment parameter now uses Unicode.
Fix make_command_line to handle Unicode correctly

Previously, make_command_line iterates over each u8 in the string and
then appends them as chars, so any non-ASCII string will get horribly
mangled by this function.  This fix should allow Unicode arguments to
work correctly in process::spawn.
Use Get/FreeEnvironmentStringsW instead of Get/FreeEnvironmentStringsA

Changed libstd to use Get/FreeEnvironmentStringsW instead of
Get/FreeEnvironmentStringsA to support Unicode environment variables.