BoxedResidue: (almost) in-place Montgomery reductions

[tarcieri]

Eliminates several allocations by performing most operations of a Montgomery reduction in-place.

Only conditionally adding the modulus presently incurs an allocation.

Benchmarks

Montgomery arithmetic/BoxedResidue creation
                        time:   [135.30 ns 135.50 ns 135.76 ns]
                        change: [-36.403% -36.104% -35.794%] (p = 0.00 < 0.05)
                        Performance has improved.

Montgomery arithmetic/BoxedResidue retrieve
                        time:   [203.59 ns 204.01 ns 204.45 ns]
                        change: [-17.533% -16.947% -16.403%] (p = 0.00 < 0.05)
                        Performance has improved.

Montgomery arithmetic/multiplication, BoxedUint*BoxedUint
                        time:   [223.19 ns 223.76 ns 224.40 ns]
                        change: [-19.576% -19.107% -18.645%] (p = 0.00 < 0.05)
                        Performance has improved.

Montgomery arithmetic/modpow, BoxedUint^BoxedUint
                        time:   [58.927 µs 59.009 µs 59.103 µs]
                        change: [-18.486% -18.227% -17.952%] (p = 0.00 < 0.05)
                        Performance has improved.