Implemented CAPI rb_enc_codepoint_len.

As I was trying to run [fast_blank](https://github.com/SamSaffron/fast_blank) on rubinius I've discovered that rb_enc_codepoint_len hadn't been implemented.
So I've looked at the other CAPIs and tried to mimic tests & implementation.
Hope me and my pair did everything right. :smiley: