Struct __init__ doesn't check input type

[cjdrake]

class MyStruct(VecStruct):
    x: Vec[4]
    y: Vec[4]

>>> MyStruct(x=vec("3b111"), y=vec("5b11111"))
MyStruct(x=Vec[4](0b00101010), y=Vec[4](0b10101010))

The first one should fail b/c it has 3 bits instead of 4. The second one should fail b/c it overflows the field.