glib/gvarianttype: g_variant_type_is_subtype_of() fastpath
This adds a fastpath for the extremely common case of checking if a GVariant type is a subtype of itself and a definite basic type. For example, checking 'i' against 'i' or 's' against 's'.
In a loop using GVariantBuilder
this can cut the cost of this function
alone in half on profiles from 3.3% of samples to 1.7% of samples.