Release zcash_primitives 0.6.0 et al

[str4d]

Closes #492.
Closes #458.

[codecov]

Codecov Report

Merging #553 (c812d36) into master (4dc0f46) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #553      +/-   ##
==========================================
+ Coverage   51.52%   51.55%   +0.02%     
==========================================
  Files          91       91              
  Lines        8500     8500              
==========================================
+ Hits         4380     4382       +2     
+ Misses       4120     4118       -2     

Impacted Files	Coverage Δ
zcash_primitives/src/block.rs	36.95% <0.00%> (-2.18%)	⬇️
zcash_proofs/src/circuit/ecc.rs	69.26% <0.00%> (ø)
zcash_primitives/src/transaction/mod.rs	34.84% <0.00%> (ø)
zcash_client_sqlite/src/wallet.rs	29.22% <0.00%> (+0.28%)	⬆️
zcash_primitives/src/transaction/tests.rs	95.52% <0.00%> (+1.49%)	⬆️
...imitives/src/transaction/components/transparent.rs	39.58% <0.00%> (+2.08%)	⬆️

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