`is` handles type aliases of `integer` incorrectly and `nil` inconsistently

[purplesyringa]

local x = nil

print(x is integer) -- lowers to math.type(x) == "integer"
print(x is nil) -- lowers to x == nil

local type A = integer
print(x is A) -- lowers to type(x) == "number"
local type B = nil
print(x is B) -- lowers to type(x) == "nil"