fix: neither uint64 or int64 safe integer

[ChrisCho-H]

Number.isSafeInteger is only true when int54.
Because here also checks whether positive, it's actually uint53 which can pass these assertions.
both int64 and uint64 wrong and misleading

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.02 ⚠️

Comparison is base (b005869) 69.55% compared to head (5228a00) 69.54%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1164      +/-   ##
==========================================
- Coverage   69.55%   69.54%   -0.02%     
==========================================
  Files         158      159       +1     
  Lines       26603    26607       +4     
==========================================
- Hits        18505    18503       -2     
- Misses       8098     8104       +6     

Impacted Files	Coverage Δ
lib/btc/amount.js	20.63% <ø> (ø)
lib/btc/uri.js	0.00% <ø> (ø)
lib/primitives/coin.js	89.85% <ø> (ø)
lib/primitives/output.js	88.63% <ø> (ø)

... and 7 files with indirect coverage changes

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[pinheadmz]

I understand where you're coming from but I don't think int53 is a real type? If we do change these messages maybe it should just just say "safe integer" specifically?

[ChrisCho-H]

It's normally not in use but can be used in some languages where u can set any bit size for int or uint.
But for javascript, as u suggested, I think safe integer would sound appropriate enough for error message(number must be below 2^53).
I've changed it.