Fix breakage from zeroize updates

[brndnmtthws]

This bumps the minimum version for zeroize to 1.6, and fixes some breakage associated with its changes.

This also fixes some deprecation warnings from base64.

This resolves #43.

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[codecov]

Codecov Report

Merging #44 (e69e158) into main (66bf005) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head e69e158 differs from pull request most recent head 9cb417f. Consider uploading reports for the commit 9cb417f to get more accurate results

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
+ Coverage   65.17%   65.19%   +0.02%     
==========================================
  Files          44       44              
  Lines        3164     3166       +2     
==========================================
+ Hits         2062     2064       +2     
  Misses       1102     1102              

Impacted Files	Coverage Δ
src/blake2b/blake2b_simd.rs	0.00% <ø> (ø)
src/blake2b/blake2b_soft.rs	89.30% <ø> (ø)
src/classic/crypto_auth.rs	95.74% <ø> (ø)
src/classic/crypto_core.rs	99.23% <ø> (ø)
src/classic/crypto_generichash.rs	75.00% <ø> (ø)
src/classic/crypto_kdf.rs	86.66% <ø> (ø)
src/classic/crypto_onetimeauth.rs	89.65% <ø> (ø)
src/classic/crypto_pwhash.rs	63.15% <ø> (ø)
src/classic/crypto_secretbox.rs	84.21% <ø> (ø)
...c/classic/crypto_secretstream_xchacha20poly1305.rs	91.97% <ø> (ø)
... and 12 more

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