LeaseSet2 support

[str4d]

Implements Prop123.

[codecov-io]

Codecov Report

Merging #68 into master will decrease coverage by 0.42%.
The diff coverage is 69.12%.

@@            Coverage Diff             @@
##           master      #68      +/-   ##
==========================================
- Coverage   53.94%   53.51%   -0.43%     
==========================================
  Files          30       31       +1     
  Lines        3711     3793      +82     
==========================================
+ Hits         2002     2030      +28     
- Misses       1709     1763      +54

Impacted Files	Coverage Δ
src/data/dest/frame.rs	61.53% <ø> (+17.94%)	⬆️
src/netdb/mod.rs	28.72% <0%> (-1.06%)	⬇️
src/crypto/mod.rs	62.05% <0%> (-0.9%)	⬇️
src/crypto/frame.rs	43.39% <0%> (-10.1%)	⬇️
src/i2np/mod.rs	64.47% <0%> (-2.65%)	⬇️
src/i2np/frame.rs	46.84% <0%> (-0.65%)	⬇️
src/data/ls2/enc/auth.rs	50.56% <50.56%> (ø)
src/data/ls2/frame.rs	63.46% <63.46%> (ø)
src/data/ls2/enc/frame.rs	81.3% <81.3%> (ø)
src/data/ls2/enc.rs	87% <87%> (ø)
... and 6 more

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