Replace LittleEndian with open source crate

[mnemonikr]

https://docs.rs/fixed-byterepr/1.0.1/fixed_byterepr/ looks like it accomplishes the same thing

[mnemonikr]

It relies on rust-lang/rust#113521. I had forgotten that I had used the generic parameter for LittleEndian because use of the associated type is restricted to nightly currently.