Allow use of Option in contracttype

[sisuresh]

What problem does your feature solve?

We should be able to include an Option member in a contracttype. Limiting it to Option<RawVal> should be fine because doing this generically might not be possible.

The error if you try to use the following struct -

the trait `From<&core::option::Option<RawVal>>` is not implemented for `ScVal`

#[derive(Clone)]
#[contracttype)]
pub struct CustomStruct {
    pub name: Symbol,
    pub salt: Option<RawVal>,
}

[graydon]

I believe this is done now, @leighmcculloch ?

[leighmcculloch]

I'm not sure. Can you test it? If it works then we're good.