ff: Add PrimeField::ReprEndianness associated type

This enables generic code to reliably operate on the bits of an encoded
field element, by converting them to and from a known (little)
endianness.

The BitAnd and Shr bounds on PrimeField are now removed, as users can
perform these operations themselves as needed.

bellman/src/groth16/tests/dummy_engine.rs

@@ -285,6 +285,7 @@ impl Default for FrRepr {
 
 impl PrimeField for Fr {
     type Repr = FrRepr;
+    type ReprEndianness = byteorder::LittleEndian;
 
     const NUM_BITS: u32 = 16;
     const CAPACITY: u32 = 15;