add todo comments for segwit migration

[tkclough]

closes #59

Just added some comments. I'm sure some of them are incorrect (e.g. some of the comments suggesting changing getAddress() to getNestedAddress()), but I figured better to mark them anyway.

[codecov]

Codecov Report

Merging #60 into master will decrease coverage by 1.69%.
The diff coverage is 77.14%.

@@            Coverage Diff            @@
##           master      #60     +/-   ##
=========================================
- Coverage   98.82%   97.13%   -1.7%     
=========================================
  Files           6        6             
  Lines         427      453     +26     
  Branches       65       81     +16     
=========================================
+ Hits          422      440     +18     
- Misses          5       10      +5     
- Partials        0        3      +3

Impacted Files	Coverage Δ
lib/net.ts	100% <100%> (+0.98%)	⬆️
lib/verify.ts	100% <100%> (ø)	⬆️
lib/txs.ts	93.22% <52.94%> (-4.54%)	⬇️

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