Rename MaybePointer to NullablePointer

[avitkauskas]

Resolves #3273 as of RFC 63

According to my research all the changes in files relates to MaybePointer and only MaybePointer.

Excerps from the old code follows as a proof of thought:

codegen.h

typedef struct compile_t
{
  const char* str_Maybe;
}

codegen.c

static void init_runtime(compile_t* c)
{
  c->str_Maybe = stringtab("MaybePointer");
}

genserialise.c

if(package == c->str_builtin)
{
  // Don't serialise MaybePointer[A]
  if(name == c->str_Maybe)
    return true;
}

gentype.c

if(package == c->str_builtin)
{
  if(name == c->str_Pointer)
    genprim_pointer_methods(c, t);
  else if(name == c->str_Maybe)
    genprim_maybe_methods(c, t);
}

ffi.c

// Parameter type is Pointer[None]
// If the argument is Pointer[A], MaybePointer[A] or USize, allow it
while(ast_id(arg_type) == TK_ARROW)
  arg_type = ast_childidx(arg_type, 1);

if(is_pointer(arg_type) ||
  is_maybe(arg_type) ||
  is_literal(arg_type, "USize"))
  return true;

reference.c

if(is_maybe(ast))
{
  // MaybePointer[A] must be bound to a struct.
  ...

subtype.c

bool is_maybe(ast_t* type)
{
  return is_literal(type, "MaybePointer");
}

gentrace.c

if(is_maybe(type))
  return TRACE_MAYBE;

[avitkauskas]

Why did freebsd-12-debug failed?

[SeanTAllen]

Thanks @avitkauskas!