Field encapsulation

[theflyingcodr]

This is making some key fields private and replacing the setting of them with other methods to reduce the scope of user error.

Added a few convenience methods in for getting output and input by index.

[codecov-commenter]

Codecov Report

Merging #26 (e430698) into v2 (7358e36) will increase coverage by 0.15%.
The diff coverage is 83.09%.

@@            Coverage Diff             @@
##               v2      #26      +/-   ##
==========================================
+ Coverage   77.81%   77.97%   +0.15%     
==========================================
  Files          18       18              
  Lines        1073     1117      +44     
==========================================
+ Hits          835      871      +36     
- Misses        160      164       +4     
- Partials       78       82       +4     

Impacted Files	Coverage Δ
signaturehash.go	84.00% <60.00%> (ø)
input.go	75.64% <66.66%> (-3.73%)	⬇️
bscript/bip276.go	79.31% <75.00%> (+5.39%)	⬆️
txinput.go	86.66% <77.77%> (-0.57%)	⬇️
tx.go	82.96% <88.63%> (+0.91%)	⬆️
bscript/addressvalidation.go	87.87% <100.00%> (-0.36%)	⬇️
bscript/script.go	60.00% <100.00%> (-0.44%)	⬇️
bscript/unlockingscript.go	100.00% <100.00%> (ø)
localsigner.go	62.50% <100.00%> (ø)
output.go	87.30% <100.00%> (ø)
... and 3 more

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