loaded_image: make load options API safer

[nicholasbishop]

The spec describes LoadOptions as "A pointer to the image’s binary
load options. See the OptionalData parameter in [section 3.1.3 Load
Options] of the Boot Manager chapter for information on the source of
the LoadOptions data." The OptionalData field is described as "The
remaining bytes in the load option descriptor are a binary data buffer
that is passed to the loaded image."

So there's no guarantee made about the contents of LoadOptions; it's
arbitrary binary data as far as the UEFI spec is concerned. So it's not
necessarily safe to treat the load_options as a Char16 pointer, since
it might not be aligned and might not be null-terminated. That said, the
data is usually a null-terminated UCS-2 string. The UEFI Shell spec
specifies that command-line parameters are passed that way.

To safely support both cases, change the load_options field to a u8
pointer, drop the load_options method that converts the data to a
&str, and add two new methods: load_options_as_bytes and
load_options_as_cstr16. The set_load_options has also been changed
to take a u8 pointer instead of a Char16 pointer.

#73

[GabrielMajeri]

Good catch, I didn't realize the UEFI spec allows for non-string extra data