chore(bigint): write with Writer for fewer allocations

[rvagg]

Parking-lot for now because I had this on a local branch and think it makes a slight difference in allocations. Probably needs some verifications though!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 53.84615% with 18 lines in your changes missing coverage. Please review.

Project coverage is 3.40%. Comparing base (d1b1091) to head (ab15a64).

Files with missing lines	Patch %	Lines
big/int.go	53.84%	11 Missing and 7 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #345   +/-   ##
=======================================
  Coverage    3.39%    3.40%           
=======================================
  Files         663      663           
  Lines      178011   178044   +33     
=======================================
+ Hits         6041     6056   +15     
- Misses     170121   170134   +13     
- Partials     1849     1854    +5     

Files with missing lines	Coverage Δ
big/int.go	63.00% <53.84%> (-2.72%)	⬇️