optimize exp for even bases

[Valeh2012]

full benchstat:

goarch: amd64
pkg: github.com/holiman/uint256
cpu: Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz
                         │    old.txt     │               new.txt               │
                         │     sec/op     │   sec/op     vs base                │
Exp/large/big-2               21.01µ ± 8%   21.60µ ± 5%        ~ (p=0.393 n=10)
Exp/large/even/big-2          6.033µ ± 6%   6.133µ ± 6%        ~ (p=0.353 n=10)
Exp/large/uint256-2           2.311µ ± 4%   2.348µ ± 5%        ~ (p=0.315 n=10)
Exp/large/even/uint256-2   2279.500n ± 4%   6.894n ± 3%  -99.70% (p=0.000 n=10)
Exp/small/big-2               7.050µ ± 8%   6.753µ ± 3%        ~ (p=0.579 n=10)
Exp/small/even/big-2          1.979µ ± 6%   1.982µ ± 6%        ~ (p=0.684 n=10)
Exp/small/uint256-2           200.0n ± 4%   205.8n ± 5%        ~ (p=0.853 n=10)
Exp/small/even/uint256-2    206.300n ± 4%   7.024n ± 5%  -96.60% (p=0.000 n=10)
geomean                       2.104µ        671.8n       -68.07%

                         │    old.txt     │                new.txt                │
                         │      B/op      │     B/op      vs base                 │
Exp/large/big-2            17.72Ki ± 0%     17.72Ki ± 0%       ~ (p=1.000 n=10) ¹
Exp/large/even/big-2       1.391Ki ± 0%     1.391Ki ± 0%       ~ (p=1.000 n=10) ¹
Exp/large/uint256-2          0.000 ± 0%       0.000 ± 0%       ~ (p=1.000 n=10) ¹
Exp/large/even/uint256-2     0.000 ± 0%       0.000 ± 0%       ~ (p=1.000 n=10) ¹
Exp/small/big-2            7.219Ki ± 0%     7.219Ki ± 0%       ~ (p=1.000 n=10) ¹
Exp/small/even/big-2       1.203Ki ± 0%     1.203Ki ± 0%       ~ (p=1.000 n=10) ¹
Exp/small/uint256-2          0.000 ± 0%       0.000 ± 0%       ~ (p=1.000 n=10) ¹
Exp/small/even/uint256-2     0.000 ± 0%       0.000 ± 0%       ~ (p=1.000 n=10) ¹
geomean                                 ²                 +0.00%                ²
¹ all samples are equal
² summaries must be >0 to compute geomean

                         │   old.txt    │               new.txt               │
                         │  allocs/op   │ allocs/op   vs base                 │
Exp/large/big-2            189.0 ± 0%     189.0 ± 0%       ~ (p=1.000 n=10) ¹
Exp/large/even/big-2       15.00 ± 0%     15.00 ± 0%       ~ (p=1.000 n=10) ¹
Exp/large/uint256-2        0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
Exp/large/even/uint256-2   0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
Exp/small/big-2            77.00 ± 0%     77.00 ± 0%       ~ (p=1.000 n=10) ¹
Exp/small/even/big-2       13.00 ± 0%     13.00 ± 0%       ~ (p=1.000 n=10) ¹
Exp/small/uint256-2        0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
Exp/small/even/uint256-2   0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹
geomean                               ²               +0.00%                ²
¹ all samples are equal
² summaries must be >0 to compute geomean

closes #188

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (555918b) to head (11b4a73).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #189   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines         1666      1675    +9     
=========================================
+ Hits          1666      1675    +9