Cleanup: impl Upcast helper for NLL typed place expression construction

[nikomatsakis]

Follow-up cleanup from PR #246:

TypedPlaceExpression struct literal construction in nll.rs requires explicit Arc::new and .clone() calls:

root: Arc::new(root_expr.clone()),
ty: ty.clone(),
kind: TypedPlaceExpressionKind::Deref(Arc::new(inner_expr.clone())),

Upcast doesn't help with struct fields, so a constructor or builder that accepts impl Upcast<T> would be needed to clean this up.

See: #246 (comment)

[nikomatsakis]

Note: I expect to tackle this as part of the NLL rewrite I'm working on.

[nikomatsakis]

@JojoFlex1 you are welcome to take on this issue or #249, I'm not doing either of them.