eliminate sxg_buffer_t from internal space

[kumagi]

On making this library useful from Ruby or Python or C++, sxg_buffer_t is too closely bounded with internal implementation, we should divide it into pure pointer and length manipulation.

[twifkak]

@kumagi Did you confirm that this is necessary for Ruby/Python bindings? Did you do the additional research you had planned? Would appreciate details if you have them.