lang: Add state size override

[armaniferrante]

Allows one to specify the size of a state account. If none is provided, uses the (current) default, i.e., whatever the size of the returned value of the new ctor (plus 8 bytes for the account discriminator).

Example:

pub const MY_SIZE: u64 = 100;

#[state(MY_SIZE)]
pub struct MyState {
...
}