feat: add PowToIntPart

[quagmt]

Description

• Add (d Decimal) PowToIntPart(e Decimal) which raises d to the power of integer part of e. This is useful when the exponent is stored in Decimal.

• Adding this method instead of IntPart() (int32,error) to use with PowInt32 to avoid making the code unnecessarily complicated when we just need to raise a number to the power of its integer part (like the example below)

     intPart, err := a.IntPart()
     if err != nil {
        // handle error
     }
  
     d, err := c.PowInt32(intPart)
     if err != nil {
       // handle error
     }

• Also add Int64() (int64,error) to return integer part if it fits int64

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.13%. Comparing base (f6d2877) to head (38e4c17).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   95.94%   96.13%   +0.19%     
==========================================
  Files           5        5              
  Lines        1849     1863      +14     
==========================================
+ Hits         1774     1791      +17     
+ Misses         50       48       -2     
+ Partials       25       24       -1     

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