MemoryError reimplemented without its own prototype.

MemoryError reimplemented without its own prototype.

MemoryError is a special preallocated immutable object.  Its value type
is NJS_OBJECT_INTERNAL_ERROR. Initially the object had its own prototype
object.  It introduced inconsistency between value types and prototype
types, because some routines (for example, njs_object_prototype_to_string())
expect them to be pairwise aligned.