mutable proto structs don't prevent type change during assingment

[BeastyBlacksmith]

MWE:

@proto mutable struct A
    F::Int
end

a = A(1)
a.F = "2" #doesn't error

[BeastyBlacksmith]

It's not so difficult to fix the concretely typed case, but also handling abstract types and type parameters requires some thought.

[Tortar]

It didn't also prevent this

@proto mutable struct A
    F::Int
end

a = A("2")

but in #19 it does :-)