Change methods to `const` where appropriate

We should do a pass of the codebase looking for opportunities to change methods to `const`. For example, [`impl Weight`](https://github.com/casper-network/casper-node/blob/dev/types/src/account.rs#L176-L186) should be changed to:

```rust
impl Weight {
/// Constructs a new `Weight`.
pub const fn new(weight: u8) -> Weight {
Weight(weight)
}

/// Returns the value of `self` as a `u8`.
pub const fn value(self) -> u8 {
self.0
}
}
```