Added big endian feature for little endian CPU

[xtmono]

Handle big endian data on little endian CPU.
Useful for dealing with network data.

[kkysen]

Can you add a test showing how (and testing that) this works and would be used? You can put them alongside the other bitfields tests in tests/. Also, can you add documentation somewhere detailing how this can be used? Thanks!

[kkysen]

Suggested change

	endian: endian,
	endian,

[kkysen]

Suggested change

	let byte_index = if big_endian {
	field.len() - 1 - (bit_index / 8)
	} else {
	bit_index / 8
	};
	let byte_index = bit_index / 8;
	let byte_index = if big_endian {
	field.len() - 1 - byte_index
	} else {
	byte_index
	};

[kkysen]

Also, can you refactor all 3 (the 2 below changes, too) of these same computations into some helper function?

[kkysen]

Should it only check for big or should it have to be big, little, or nothing?

[chrysn]

endian could be an enum, and if it had a default, this could be .unwrap_or_default(). (I lack the context here to understand whether a default endianness makes sense at all; gut says no).

[kkysen]

Suggested change

	let field_endians: Vec<_> = bitfields
	.iter()
	.map(|field| {
	field
	.endian
	.as_ref()
	.map_or(false, |endian| endian == "big")
	})
	.collect();
	let field_endians = bitfields
	.iter()
	.map(|field| {
	field
	.endian
	.as_ref()
	.map_or(false, |endian| endian == "big")
	})
	.collect::<Vec<_>>();