Make SecretKey::try_from_be_bytes() take just a slice reference

[fjarri]

Type of PR:

• Feature

Required reviews:

• 1

What this does:

• Make SecretKey::try_from_be_bytes() take just a slice reference; it's the caller's responsibility to ensure that the storage is zeroized.
• Bump MSRV to 1.70

This overrides #132.

[codecov-commenter]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (b3eaa7a) 58.11% compared to head (9a2ee66) 58.21%.

Files	Patch %	Lines
umbral-pre/src/keys.rs	0.00%	5 Missing ⚠️
umbral-pre/src/bindings_python.rs	0.00%	1 Missing ⚠️
umbral-pre/src/bindings_wasm.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #134      +/-   ##
==========================================
+ Coverage   58.11%   58.21%   +0.09%     
==========================================
  Files          17       17              
  Lines        3123     3118       -5     
==========================================
  Hits         1815     1815              
+ Misses       1308     1303       -5     

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