Speed up encoding.

[jondeandres]

Seems our Encoding and Encode classes were quite slow, I think I've speed-up it a bit. These are my benchmark results:

Iterations: 10

master branch

                           user     system      total        real
ascii_chars            5.610000   0.010000   5.620000 (  5.633157)
iso_8859_1             5.630000   0.000000   5.630000 (  5.633440)
utf8                   5.350000   0.010000   5.360000 (  5.361992)
russian_iso_8859_1     5.930000   0.000000   5.930000 (  5.941858)

speed-up-encoding branch

                           user     system      total        real
ascii_chars            0.600000   0.000000   0.600000 (  0.594938)
iso_8859_1             0.600000   0.000000   0.600000 (  0.607980)
utf8                   0.400000   0.010000   0.410000 (  0.408550)
russian_iso_8859_1     0.650000   0.000000   0.650000 (  0.642910)