Keep excluding the BigMath.log(42) for now

* performance, takes 0.4 seconds
Offset the calculation to efficient (0.1)...(10) window

* use the exponent as lwo cost window definition
* the MRI code used exponent > 2 (but that needed going up
  to 99 without offsetting); using the (x > 10) here, allows
  to offset any value > 10 (that is ]10,100[) also, gaining
  speed on the tests for 42, 99 etc.

Some performance results (MacBook Pro)

2.0.0-p247 :046 > Benchmark.bmbm do |bm|
2.0.0-p247 :047 >     bm.report('1') { BigMath.log(1, 20) }
2.0.0-p247 :048?>     bm.report('9') { BigMath.log(9, 20) }
2.0.0-p247 :049?>     bm.report('11') { BigMath.log(11, 20) }
2.0.0-p247 :050?>     bm.report('99') { BigMath.log(99, 20) }
2.0.0-p247 :051?>     bm.report('999') { BigMath.log(999, 20) }
2.0.0-p247 :052?>     bm.report('999_999_999') { BigMath.log(999_999_999, 20) }
2.0.0-p247 :053?>     bm.report('0.9') { BigMath.log(BigDecimal.new('0.9'), 20) }
2.0.0-p247 :054?>     bm.report('0.09') { BigMath.log(BigDecimal.new('0.09'), 20) }
2.0.0-p247 :055?>     bm.report('0.000_000_009') { BigMath.log(BigDecimal.new('0.000_000_009'), 20) }
2.0.0-p247 :056?>   end
Rehearsal -------------------------------------------------
1               0.000000   0.000000   0.000000 (  0.001000)
9               0.030000   0.000000   0.030000 (  0.025000)
11              0.110000   0.020000   0.130000 (  0.093000)
99              0.090000   0.010000   0.100000 (  0.038000)
999             0.100000   0.010000   0.110000 (  0.037000)
999_999_999     0.110000   0.010000   0.120000 (  0.039000)
0.9             0.010000   0.000000   0.010000 (  0.005000)
0.09            0.110000   0.010000   0.120000 (  0.036000)
0.000_000_009   0.090000   0.000000   0.090000 (  0.032000)
---------------------------------------- total: 0.710000sec

                    user     system      total        real
1               0.000000   0.000000   0.000000 (  0.001000)
9               0.070000   0.000000   0.070000 (  0.022000)
11              0.090000   0.000000   0.090000 (  0.032000)
99              0.050000   0.000000   0.050000 (  0.014000)
999             0.040000   0.000000   0.040000 (  0.013000)
999_999_999     0.040000   0.000000   0.040000 (  0.012000)
0.9             0.000000   0.000000   0.000000 (  0.001000)
0.09            0.030000   0.000000   0.030000 (  0.012000)
0.000_000_009   0.030000   0.000000   0.030000 (  0.013000)
Remove excludes of most :test_BigMath_log tests