MaxEncodedLen macro is not accurate for compact fields, and skipped fields and variants

AFAICT MaxEncodedLen doesn't read the attributes `#[codec(skip)]` and `#[codec(encode_as..)]` and `#[codec(compact)]`.

Thus it can result in an inaccurate len.

For instance the type:
```
#[derive(MaxEncodedLen, Encode)]
struct Foo(#[codec(compact)] u8);
```
will have max encoded len of 1 but can actually encode in 2 bytes.