Simplify BergValue interface

[jkeiser]

Replace infix, prefix, etc. with one method:

pub trait BergValue<'a> {
    ...
    /// Perform an operation on this BergValue, and return the result as well
    /// as the updated BergValue.
    /// 
    /// Returns:
    /// - `Err(error)` if there was an error performing this operation.
    /// - `Ok((current, new_self))` if the operation worked out fine.
    fn operate(self, operation: Operation<impl BergValue<'a>>) -> BergResult<'a, (BergResult<'a>, BergResult<'a>)>;
}

pub enum Operation<Operand> {
    Infix { operator: IdentifierIndex, is_assign: bool, right: Operand },
    Postfix { operator: IdentifierIndex },
    Prefix { operator: IdentifierIndex },
    SubExpression { boundary: ExpressionBoundary },
    Field { field: IdentifierIndex },
    SetField { field: IdentifierIndex, value: Operand },
    NextVal
}

The intent is threefold:

• Make lazy values and proxies easier (since they are so prevalent)
• Unify all operations so they follow the same pattern, so that typechecking will be clearer
• Simplify BergValue implementors