Use dynamic precision convertations in inner ring

[alexvanin]

Related to #122

[codecov]

Codecov Report

Merging #126 into master will increase coverage by 0.50%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #126      +/-   ##
==========================================
+ Coverage   44.11%   44.62%   +0.50%     
==========================================
  Files          52       53       +1     
  Lines        2525     2548      +23     
==========================================
+ Hits         1114     1137      +23     
  Misses       1291     1291              
  Partials      120      120              

Impacted Files	Coverage Δ
pkg/util/precision/converter.go	100.00% <100.00%> (ø)

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[alexvanin]

Added precision.Convert() for in-place conversions. This could be useful in accounting.Balance when big.Int value from RPC node should be packed into Decimal with int64 value field.