convert some typeof tags to small integers
The value here is two-fold. One, we speed up sysimg load times slightly,
since we can entirely skip relocations for these values and avoid
faulting in some writable pages. This should let us (later) move more
content into the ConstData section, such as String objects. Secondly,
some type-manipulation native code becomes simpler, since it is based on
small consecutive constants instead of requiring extra pointer loads to
check for pointer identity. This is similar to the existing small-union
optimization, but for type tags, and only for specific ones.
This makes use of the fact that the zero page (about 4096 byes) cannot
be allocated in the usual C memory model, which lets us define up to 255
of these special types, after taking into account the 4 gc bits. As a
current implementation limitation for performance, these cannot be
parametric types.
Note there are probably more places in Base that can be updated to use
`jl_typetagof` instead of `jl_typeof`, where we know if it is a type or tag.
For example, this optimize most of builtins.c file, except for the
`jl_object_id` function.
