Increase performance of :two_matrix_rows about 7% for MRI

Hello again!

Here we have another one performance increase for MRI.

My tests showed that indexing a String is slower than indexing an Array.

Since we index the "from" and "to" strings inside big and nested iterations it ended up 7% faster converting those 2 strings to array of characters. Replacing #fetch with square brackets for the indexing of options also helped a bit.

For jruby and rbx this optimizations does not work (again).

And the benchmarks: 
(IterativeWithTwoMatrixRowsOptimizedMore was temporary class with the new changes in order to make more representative benchmarks)


Rehearsal ---------------------------------------------------------------------------------------------------------------------
StringMetric::Levenshtein::Experiment implementation                                2.100000   0.010000   2.110000 (  2.143954)
StringMetric::Levenshtein::IterativeWithFullMatrix implementation                   2.340000   0.010000   2.350000 (  2.379848)
StringMetric::Levenshtein::IterativeWithTwoMatrixRows implementation                2.060000   0.010000   2.070000 (  2.102046)
StringMetric::Levenshtein::IterativeWithTwoMatrixRowsOptimized implementation       1.710000   0.000000   1.710000 (  1.720389)
StringMetric::Levenshtein::IterativeWithTwoMatrixRowsOptimizedMore implementation   1.590000   0.000000   1.590000 (  1.583849)
Text::Levenshtein implementation                                                    2.260000   0.000000   2.260000 (  2.269622)
----------------------------------------------------------------------------------------------------------- total: 12.090000sec

                                                                                        user     system      total        real
StringMetric::Levenshtein::Experiment implementation                                2.100000   0.000000   2.100000 (  2.095981)
StringMetric::Levenshtein::IterativeWithFullMatrix implementation                   2.300000   0.000000   2.300000 (  2.296880)
StringMetric::Levenshtein::IterativeWithTwoMatrixRows implementation                2.010000   0.000000   2.010000 (  2.016962)
StringMetric::Levenshtein::IterativeWithTwoMatrixRowsOptimized implementation       1.700000   0.010000   1.710000 (  1.713488)
StringMetric::Levenshtein::IterativeWithTwoMatrixRowsOptimizedMore implementation   1.570000   0.000000   1.570000 (  1.563868)
Text::Levenshtein implementation                                                    2.210000   0.000000   2.210000 (  2.209920)