change type none to type missing and improve type safety

[mccanne]

This commit addresses some confusion that has been arising regarding the difference between option types and a union (none|T). To do so, we renamed the none type as missing and introduce the concept that there are two types of nones: a missing none (i.e., none::missing), and an option none (i.e., none::option(T) aka none::T).

none::missing values can be cast to option(T) by casting the none to type T. While none::missing exists in the runtime as a transient value, they are not expected to be serialized anywhere (except for inside fusion values). That said, we allow their serialization for test and debug with the goal of making sure the runtime has sufficient type safety so that they can never be materialized, e.g., "values none" (which implies none::missing) should produce an error though we haven't implemented this yet. Option nones are always serializable.

Fusion now creates fused records with missing fields with the missing type. This seems to be less confusing.

The option type now includes vector.Missing to represent none case but we change the conventions so that the these none::missing values are never expressed out of an option value. Instead, option.Coalesce is called with default values or errors are expressed as nones.

We changed Apply so RipFusions does not rip options and RipOption continues to mean to separate the nones (as missings) and the somes. It's up to the caller of Apply to handle the option values appropriately. This moves that logic to the call sites where type-safe semantics can be implemented requiring users to specify queries that handle the none cases when they arise. (There are more cases to cover in the runtime, and the next step with the type checker is to catch at compile time as many as the option-handling type errors runtime as possiblea)