Empty zcash_note_encryption crate

[str4d]

The general note encryption logic shared between Sapling and Orchard will be moved into here.

[codecov]

Codecov Report

Merging #357 (b7c31e3) into master (3a8e729) will decrease coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #357      +/-   ##
==========================================
- Coverage   64.64%   64.58%   -0.06%     
==========================================
  Files          69       70       +1     
  Lines        7079     7082       +3     
==========================================
- Hits         4576     4574       -2     
- Misses       2503     2508       +5     

Impacted Files	Coverage Δ
components/zcash_note_encryption/src/lib.rs	0.00% <0.00%> (ø)
zcash_primitives/src/block.rs	45.16% <0.00%> (-1.62%)	⬇️
zcash_primitives/src/legacy.rs	60.81% <0.00%> (-0.84%)	⬇️
zcash_primitives/src/transaction/mod.rs	48.86% <0.00%> (-0.38%)	⬇️
zcash_primitives/src/note_encryption.rs	76.17% <0.00%> (ø)
zcash_proofs/src/circuit/sprout/input.rs	0.00% <0.00%> (ø)
zcash_primitives/src/transaction/components.rs	50.59% <0.00%> (ø)

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[nuttycom]

Incorporated into #358

[str4d]

I prefer to merge this PR and then migrate the logic in a separate PR.