Fix: Use varadic call internally

[tigh-latte]

There's no point iterating the utxos in tx.Fund and passing them into FromUTXOs when we can just pass them in at once.

[codecov-commenter]

Codecov Report

Merging #61 (504e664) into master (7440322) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   84.76%   84.75%   -0.01%     
==========================================
  Files          28       28              
  Lines        3012     3011       -1     
==========================================
- Hits         2553     2552       -1     
  Misses        325      325              
  Partials      134      134              

Impacted Files	Coverage Δ
txinput.go	85.26% <0.00%> (-0.16%)	⬇️

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