Refactored njs_vmcode_interpreter() for performance.

Refactored njs_vmcode_interpreter() for performance.

    1) opcodes are rewritten using switch table.
    2) often-used opcodes are prioritized and
       inlined.
    3) similar opcodes are combined into unified
       handlers.
    4) njs_vmcode_interpreter() return NJS_OK on
       success. NJS_STOP return code is removed.