implement Log10 and CmpUint64

[holiman]

Work in progress

Adds the following methods

// CmpUint64 compares z and x and returns:
//
//	-1 if z <  x
//	 0 if z == x
//	+1 if z >  x
func (z *Int) CmpUint64(n uint64) int 

// Log10 returns the log in base 10, floored to nearest integer.
// **OBS** This method returns '0' for '0', not `-Inf`.
func (z *Int) Log10() uint

[codecov]

Codecov Report

Merging #134 (f93e900) into master (71f8c05) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #134   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            5         5           
  Lines         1615      1671   +56     
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+ Hits          1615      1671   +56