Better handling for `len` attribute in vk.xml

[achirkin]

vulkan/genvulkan/src/VkXml/Sections/Types.hs

Lines 210 to 214 in 84a01f4

	parseAttrVkMemberLen :: ReaderT ParseLoc AttrParser (Maybe Text)
	parseAttrVkMemberLen = lift $ do
	maltlen <- attr "altlen"
	mlen <- attr "len"
	pure $ maltlen <|> mlen

Make len and altlen parsing more correct and robust w.r.t. spec 10.2.2 "attributes of member tags".