fix electrum floating-point arithmetic issue

[zoedberg]

Closes #62

Issue was due to the conversion from float to integer in let value = (vout.value * 100_000_000.0) as u64;

With this PR instead of parsing the TX outputs of the JSON TX returned by electrum we use the TX deserialized from its hex, this way we don't have to deal with floating-point arithmetic issues.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Project coverage is 0.0%. Comparing base (dbfb8ba) to head (81346f6).
Report is 3 commits behind head on master.

Files	Patch %	Lines
src/indexers/electrum.rs	0.0%	9 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##           master    #61   +/-   ##
=====================================
  Coverage     0.0%   0.0%           
=====================================
  Files          23     23           
  Lines        2128   2123    -5     
=====================================
+ Misses       2128   2123    -5     

Flag	Coverage Δ
rust	0.0% <0.0%> (ø)

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[dr-orlovsky]

Closing in favor of merged #63