aes-siv: bump aes crate to v0.8; MSRV 1.56+

[tarcieri]

No description provided.

[tarcieri]

I'm still not exactly sure why but I had to disambiguate these:

   Compiling aes-siv v0.7.0-pre (/Users/tony/src/RustCrypto/AEADs/aes-siv)
error[E0034]: multiple applicable items in scope
   --> aes-siv/src/siv.rs:275:9
    |
275 |     mac.update(&Tag::default());
    |         ^^^^^^ multiple `update` found
    |
note: candidate #1 is defined in the trait `Mac`
   --> /Users/tony/.cargo/registry/src/github.com-1ecc6299db9ec823/digest-0.10.3/src/mac.rs:38:5
    |
38  |     fn update(&mut self, data: &[u8]);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: candidate #2 is defined in the trait `Update`
   --> /Users/tony/.cargo/registry/src/github.com-1ecc6299db9ec823/digest-0.10.3/src/lib.rs:77:5
    |
77  |     fn update(&mut self, data: &[u8]);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: disambiguate the associated function for candidate #1
    |
275 |     Mac::update(&mut mac, &Tag::default());
    |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
help: disambiguate the associated function for candidate #2
    |
275 |     Update::update(&mut mac, &Tag::default());
    |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...which occurs even if the Update trait hasn't been imported or bounded upon, nor is it a bound of Mac, so I'm not sure why this is ambiguous

cc @newpavlov

[codecov-commenter]

Codecov Report

Merging #431 (e4a7e30) into master (6b980d9) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #431      +/-   ##
==========================================
- Coverage   87.38%   87.28%   -0.10%     
==========================================
  Files          40       40              
  Lines        1958     1959       +1     
==========================================
- Hits         1711     1710       -1     
- Misses        247      249       +2     

Impacted Files	Coverage Δ
aes-siv/src/lib.rs	77.77% <ø> (ø)
aes-siv/src/siv.rs	94.11% <100.00%> (-2.90%)	⬇️

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