Maybe support inline type hint

[gyson]

We should already be able to do type hint with T.assert( expr :: type ). For example,

map = %{} 

# type is empty map

T.assert(map :: %{optional(String.t) => integer})

# type is updated to %{optional(String.t) => integer} now.

if we support inline type hint like expr ~> type annotation. it may be easier (syntax sugar) to do type declaration, especially in pattern match.

we can do

{first ~> integer(), second ~> String.t} = something_untyped

instead of

{first, second} = something_untyped
T.assert first :: integer
T.assert second :: String.t

^^ benefit is to avoid write first and second twice.

Implementation concern

need to do some hacky thing to make it work. not sure if it worths the cost.