Added compilable memory.c, readparse.c
Compilable interp.c
env.c, native.c
Compilable native.c
All major .c files compiliable
Added machine.c, readparse.h
secd.c
Makefile
Refactored SECD works
Refactored out the need for cell_secd()
Abstracted input into secd_stream_t
libsecd clause for Makefile
Linux compilation fixed
secd->nil(removed) => SECD_NIL(NULL), is_error()

This commit changes:
1) SECD_NIL is now NULL, not removed secd->nil;
2) if native code fails, it now returns CELL_ERROR, not NULLs;
3) enhanced memory layout (anticipating arrays introduction);
4) some additional errors;
Correct decreasing of secd->fixedptr

Now if there are free cells adjacent to the free space,
that cells are correctly released into the free space.
Memdebug now includes information free_cells.
Enhanced (secdctl 'free)
(secdctl 'tick) now returns a number instead of printing
Lookup speed optimization