fix: use :erlang.bxor/2 instead of Bitwise.bxor/2

[the-mikedavis]

Looks like this function used to be a macro in elixir < 1.10...

michael@mango ~> nix-shell -p elixir_1_9
these paths will be fetched (3.29 MiB download, 5.37 MiB unpacked):
  /nix/store/ds8swb2gz7wmlncwm3bx8aj7mxrxkngi-elixir-1.9.4
copying path '/nix/store/ds8swb2gz7wmlncwm3bx8aj7mxrxkngi-elixir-1.9.4' from 'https://cache.nixos.org'...

[nix-shell:~]$ iex
Erlang/OTP 23 [erts-11.2.2] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]

Interactive Elixir (1.9.4) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> Bitwise.__info__(:functions)
[]
iex(2)> Bitwise.__info__(:macros)   
[
  &&&: 2,
  <<<: 2,
  >>>: 2,
  ^^^: 2,
  __using__: 1,
  band: 2,
  bnot: 1,
  bor: 2,
  bsl: 2,
  bsr: 2,
  bxor: 2,
  |||: 2,
  ~~~: 1
]
iex(3)> 
BREAK: (a)bort (A)bort with dump (c)ontinue (p)roc info (i)nfo
       (l)oaded (v)ersion (k)ill (D)b-tables (d)istribution
^C
[nix-shell:~]$ exit
michael@mango ~> nix-shell -p elixir_1_10
these paths will be fetched (3.53 MiB download, 5.89 MiB unpacked):
  /nix/store/xgfbgv3lr9vp6jsqr34pk8bq2dviazsx-elixir-1.10.4
copying path '/nix/store/xgfbgv3lr9vp6jsqr34pk8bq2dviazsx-elixir-1.10.4' from 'https://cache.nixos.org'...

[nix-shell:~]$ iex
Erlang/OTP 23 [erts-11.2.2] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]

Interactive Elixir (1.10.4) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> Bitwise.__info__(:functions)
[
  &&&: 2,
  <<<: 2,
  >>>: 2,
  ^^^: 2,
  band: 2,
  bnot: 1,
  bor: 2,
  bsl: 2,
  bsr: 2,
  bxor: 2,
  |||: 2,
  ~~~: 1
]
iex(2)> Bitwise.__info__(:macros)   
[__using__: 1]

Here's the commit that changed it from macros to functions: elixir-lang/elixir@1e4e05e#diff-032f258b047753a20ce7dbe83d825bbf440b74bae27967d6f06eda11d379680e

No need to die on this hill and declare only ~> 1.10 compatibility: might as well just call the erlang functions like the stdlib does