Add a MaxSize trait and derive macro

[lachlansneff]

This adds a trait MaxSize:

pub trait MaxSize {
    /// The maximum possible size that the serialization of this
    /// type can have, in bytes.
    const POSTCARD_MAX_SIZE: usize;
}

and a derive macro that implements it for structs and enums.

I think this just needs a little more documentation and then it'll be ready to merge. I would like someone else to double check my logic for computing the varint size though.

The naming needs a little more work too. I'm not super happy with the trait and constant name.

[lachlansneff]

Here's what the error looks like when the type of a field doesn't implement postcard::MaxSize.

error[E0277]: the size for values of type `str` cannot be known at compilation time
  --> tests/max_size.rs:62:9
   |
62 |         a: &'static str,
   |         ^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `str`
   = note: required because of the requirements on the impl of `MaxSize` for `&'static str`

error[E0277]: the trait bound `str: MaxSize` is not satisfied
  --> tests/max_size.rs:62:9
   |
62 |         a: &'static str,
   |         ^ the trait `MaxSize` is not implemented for `str`
   |
   = note: required because of the requirements on the impl of `MaxSize` for `&'static str`

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