Refactoring of string internals for vectorization

[vadim.godunko]

Performance of string processing can be significantly improved on modern CPU by using SIMD instructions. This ticket is opened to review current implementation and make it compatible with SIMD algorithms. Several significant assumptions must be defined to make easy the use of SIMD instructions.

1. Data must be aligned at alignment of largest vector supported by CPU.
2. Then string is not occupy all elements of last vector then all unused elements must be filled by zero (this may be removed actually, need to be investigated)

[vadim.godunko]

(In [255]) Move Fill_Null_Terminator into the SIMD child package.

Provide platform specific implementation for x86_64.

Refs #2.

[vadim.godunko]

(In [256]) Refactor compare operations into SIMD package and provide their
implementation in clean Ada and using x86_64 SSE2 instructions.

Refs #2.

[vadim.godunko]

(In [257]) Replace compare using SIMD instructions by ordinary code, save SIMD only
to detect difference. This improve total speed of compare operations.

Refs #2.

[vadim.godunko]

(In [258]) v8hi_Array is large as reasonable now.

Refs #2.