Left shift overflow when calculating max_degree

[dark64]

bellman/src/domain.rs

Line 64 in 3aa6226

let max_degree = (1 << E::Fr::S) - 1;

bellman/src/domain.rs

Line 116 in 3aa6226

let max_degree = (1 << E::Fr::S) - 1;

This code fails when compiled to wasm with

panicked at 'attempt to shift left with overflow', 
/home/darko/.cargo/registry/src/github.com-1ecc6299db9ec823/bellman_ce-0.3.5/src/domain.rs:64:26

when E::Fr::S is greater or equal to 32, which seems to be the case with curve bls12_381.