Type inference (synthesize) for lambdas

[hugomg]

It would be nice if we could infer the type of a lambda, by putting type annotations on the arguments:

-- infer type "integer -> integer"
local f = function(x: integer) return x+1 end

However, we still want to allow the programmer to omit the type annotations for the arguments, if the function is used in a context where the type is known:

foreach(xs, function(x)
    dosomething(x)
end)

One thing that I don't know how we should handle in the code, is if a lambda is used in a context of type any. If the type annotations are present, it should be possible to insert the type coercion. However, if the annotations are not present, then I guess it would have to be a type error.

local f: any = function(x) end