Use G_DEFINE_BOXED_TYPE_WITH_CODE to register numeric typeinfo
counterpart lazily in the *_get_type routine.

Reuse GLib byte order macros since our definitions are at best
redundant.

Let numeric_type_register_static take the NumericTypeInfo struct as
argument.

Move registration of numeric counterparts of GLib fundamental types into
numeric_get_type_info().

Mark the TYPE_TABLE symbol as static.

Remove _get_name, _get_type and _get_byte_order in favour of using the
now public NumericTypeInfo struct members directly.

It will be useful later on when we integrate dynamic types.