Vec::Buffers: new constructors: zero extend, combined
Vec::Buffers: new constructors: zero extend, combined

1. Clarify available constructor methods of vec::Buffers in an enum:
- NO_ALLOC: do not allocate any memory
- SLICE_ALLOC: allocate only a vector of pointers each points to an
    allocated memory
- ZERO_EXTEND: allocate a zero buffers and push back to the vector memory
- COMBINED: allocate only a vector of pointers that are combined from
    two vectors of two input buffers vector
- FULL_ALLOC: fully allocate memory including a vector of pointers each
    for a memory of `size` elements

2. Adding two new constructors of vec::Buffers:
- ZERO_EXTEND
- COMBINED

vrancurel/ntl,vrancurel/ntl,vrancurel/ntl