Add helper to temporarily switch endianness

[Schamper]

Sometimes we have to dynamically switch endianness for the lifetime of a parser (not the lifetime of the global cstruct instance). It would be nice to easily have the ability to switch the endianness for either a single struct parsing or of a "shallow" clone of the cstruct instance.

Maybe #140 is already "easy enough", unsure. Open to suggestions.

[yunzheng]

Maybe a contextmanager would be nice, implemented like this after merging #140:

class cstruct:
    ...
    @contextmanager
    def endianness(self, byte_order: str) -> Iterator[cstruct]:
        new = self.copy()
        new.endian = byte_order
        yield new

Example usage:

from dissect.cstruct import cstruct

data = bytes.fromhex("AABBCCDD")

cdef = """
struct test {
    uint32  a;
};
"""

cs = cstruct(cdef, endian="<")

print(cs.test(data))

with cs.endianness(">") as big:
    print(big.test(data))

Outputs:

<test a=0xddccbbaa>
<test a=0xaabbccdd>

And another convenience method like big_endian() and little_endian() could also be added like this:

    big_endian = partialmethod(endianness, byte_order=">")
    little_endian = partialmethod(endianness, byte_order="<")

[Schamper]

I was thinking something in the direction of a context manager, that's a very nice approach!

Another idea I had could be cs.test(data, endian="<"). Although that would be a lot harder change to make.