Release 0.4.0

[fracek]

• Cleanup unused imports
• Clippy fixes
• Add pre authorized tx and hash(x) signer keys
• Change transaction builder

[codecov[bot]]

Codecov Report

Merging #1 into master will increase coverage by 0.47%. The diff coverage is 78.78%.

@@            Coverage Diff             @@
##           master       #1      +/-   ##
==========================================
+ Coverage   74.17%   74.65%   +0.47%     
==========================================
  Files          29       27       -2     
  Lines        3365     3377      +12     
==========================================
+ Hits         2496     2521      +25     
+ Misses        869      856      -13     

Impacted Files	Coverage Δ
src/account.rs	75.00% <ø> (ø)
src/amount.rs	80.00% <ø> (-0.12%)	:arrow_down:
src/crypto/mod.rs	71.42% <ø> (ø)
src/operations/mod.rs	40.00% <0.00%> (-2.27%)	:arrow_down:
src/signature.rs	75.35% <ø> (ø)
src/time_bounds.rs	93.18% <ø> (ø)
src/transaction.rs	50.44% <13.63%> (-1.71%)	:arrow_down:
src/operations/allow_trust.rs	73.21% <80.00%> (+0.64%)	:arrow_up:
src/asset.rs	75.86% <100.00%> (-0.41%)	:arrow_down:
src/crypto/strkey.rs	100.00% <100.00%> (+7.54%)	:arrow_up:
... and 16 more

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