mpc-circuits byte-order/endianness

[sinui0]

Our current circuit model requires that all inputs and outputs be in little-endian and lsb0. It would be more ergonomic to allow the user to specify this, rather than forcing them to do order conversions on everything.

[themighty1]

I think we should have big-endian by default for better UX because this is the default format all other circuit builder's components will be in like e.g. sha hashes and aes keys.

[sinui0]

So, allowing the User to specify the internal representation of bit order is not required.

With #212 , the circuit model stores values in LSB0, but that detail is not exposed to the User and byte-order is totally up to them. All our circuits have been updated to use big-endian.