Define isLit for Bundle Literals

#1057 (bundle literals) is being merged as-is, where isLit on Bundles returns false. That may not be desired, though doing the opposite may break existing APIs (since isLit == true is probably followed by litArg, which would not exist for Bundle literals). Question is whether to make that work (and if not, how to define isLit - perhaps as 'is a numeric literal'?), or to recommend a different API for accessing Bundle literals such as one that returns an Option.

Also to be discussed would be how to support other Aggregate literal types that may come in the future, like Vec literals.

**Type of issue**: feature request

**Impact**: no functional change | API addition (no impact on existing code) | API modification | unknown

**Development Phase**: tracking

**Other information**

Start by reading issue #1057 and the current Bundle literal behavior around isLit and litArg. Clarify whether isLit should include aggregate literals, whether an Option-based accessor is needed, and how future Vec literals should fit; the issue is done when the API semantics and compatibility impact are decided.

Written by the indexing model from the issue text.