Fix deprecation warning for use Bitwise in Elixir 1.14+

[darraghenright]

Hi there 👋 Nice library!

I noticed a couple of deprecation warnings when running tests in a recent version:

mix test
Compiling 2 files (.ex)
warning: use Bitwise is deprecated. import Bitwise instead
  lib/utils/der.ex:4: EllipticCurve.Utils.Der (module)

warning: use Bitwise is deprecated. import Bitwise instead
  lib/utils/integer.ex:4: EllipticCurve.Utils.Integer (module)

.............
Finished in 0.06 seconds (0.00s async, 0.06s sync)
13 tests, 0 failures

I looked through the Elixir changelog and discovered that use Bitwise is hard deprecated since 1.14.

This PR makes the recommended changes and imports the module instead.

---

On another topic — I wanted to check the backwards compatibility of this change. I started by running the tests against version 1.9, which is the minimum stated compatible version in mix.exs but I encountered a compilation error:

== Compilation error in file lib/math.ex ==
** (CompileError) lib/math.ex:245: cannot invoke remote function p.y/0 inside guards
    (stdlib) lists.erl:1354: :lists.mapfoldl/3

Which refers to the following line:

defp jacobianMultiply(p, _n, _cN, _cA, _cP) when p.y == 0 do

I decided to run tests against the master branch in a number of hexpm/elixir containers and the earliest tag that did not raise this compilation error is 1.11.0-erlang-21.0.1-debian-bullseye-20210902.

I can confirm the Bitwise changes in this PR also result in a successful test run:

root@94fa891ce1c3:/srv# elixir -v
Erlang/OTP 21 [erts-10.0.1] [source] [64-bit] [smp:6:6] [ds:6:6:10] [async-threads:1]

Elixir 1.11.0 (compiled with Erlang/OTP 21)
root@94fa891ce1c3:/srv# mix test
.............

Finished in 0.1 seconds
13 tests, 0 failures

Randomized with seed 438621

So in addition to that change, I think it might be necessary to bump the minimum stated compatible version to 1.11. In any case, this is the minimum supported version of Elixir (security patches only).

[darraghenright]

Gently bumping this 👋

[darraghenright]

Is this project abandoned?