Implement CmpBig

[holiman]

This PR implements

// CmpBig compares z and x and returns:
//
//	-1 if z <  x
//	 0 if z == x
//	+1 if z >  x
func (z *Int) CmpBig(x *big.Int) (r int) 

After some benchmarking, it turned out that converting from big to Int was not significantly slower than e.g pre-checking the BitLength, so I went with that in the end.

Closes #135

[codecov]

Codecov Report

Merging #138 (93914f7) into master (71f8c05) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #138   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines         1615      1622    +7     
=========================================
+ Hits          1615      1622    +7     

[holiman]

From CI run

BenchmarkCmpBig/nonequal-36               	82800283	        16.19 ns/op	       0 B/op	       0 allocs/op
BenchmarkCmpBig/equal-36                  	126040651	        10.66 ns/op	       0 B/op	       0 allocs/op