Refactored file hierarchy.

Refactored file hierarchy.

1) all source files are moved to src directory.
2) nxt files are renamed with "njs" prefix.
3) some files are renamed to avoid collisions:
    nxt_array.c -> njs_arr.c
    nxt_array.h -> njs_arr.h
    nxt_string.h -> njs_str.h
    nxt_time.c -> njs_time.c
    nxt_time.h -> njs_time.h
    njs_time.c -> njs_timer.c
    njs_time.h -> njs_timer.c
    njs_core.h -> njs_main.h
4) C tests are moved to src/test dir.
5) Other tests are moved to test dir.
6) Some structs are renamed to avoid collisions:
    nxt_array_t -> njs_arr_t
    nxt_string_t -> njs_str_t

    appropriate functions and macros are also renamed.

7) all macros, functions and other identifiers with "NXT_" and "nxt_"
    prefixes are renamed to corresponding "NJS_" or "njs_" prefix.

NO functional changes.