Add AES-GCM-SIV support

[dewyatt]

This is based on work I did a month or so ago and improved on, fixed bugs, etc today.

• Adds a cipher module that cleans things up a bit.
• Adds support for aes-256-gcm-siv via the aes-gcm-siv crate (until botan has native support).
• Makes things a little more consistent on using our internal alg names vs botan's.
• Breaking Change: This changes the CLI options for PBKDF2. Previously you would use --pbkdf pbkdf2 --pbkdf2-hash sha256, but now you must use --pbkdf pbkdf2-sha256.

Fixes #13

[codecov[bot]]

Codecov Report

Merging #45 into master will increase coverage by 1.11%. The diff coverage is 71.2%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
+ Coverage   66.19%   67.31%   +1.11%     
==========================================
  Files           8        9       +1     
  Lines        1133     1181      +48     
  Branches      248      258      +10     
==========================================
+ Hits          750      795      +45     
+ Misses        176      164      -12     
- Partials      207      222      +15

Impacted Files	Coverage Δ
src/etree.rs	65.25% <ø> (-0.26%)	:arrow_down:
src/cas.rs	26.78% <0%> (ø)	:arrow_up:
src/lib.rs	78.82% <100%> (+3.55%)	:arrow_up:
src/crypto.rs	64.35% <64.7%> (-3.09%)	:arrow_down:
src/prot.rs	63.04% <70%> (+1.58%)	:arrow_up:
src/cipher.rs	73.48% <73.48%> (ø)
src/pbkdf.rs	76.69% <75%> (+4.23%)	:arrow_up:

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