Use CreateProcessW instead of CreateProcessA

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

Added a helper function os::win32::as_mut_utf16_p, which does basically
the same thing as os::win32::as_utf16_p except the pointer is mutable.

See issue #13815.
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 native::io::process::spawn.

See issue #13815.
Use Get/FreeEnvironmentStringsW instead of Get/FreeEnvironmentStringsA

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

Added a run-pass test to ensure that processes can be correctly spawned
using non-ASCII arguments, working directory, and environment variables.
It also tests Unicode support of os::env_as_bytes.

An additional assertion was added to the test for make_command_line to
verify it handles Unicode correctly.

See issue #13815.