Make `RefCtx` less ubiquitous internally

[jorisdral]

Since #781, sessions internally store a RefCtx and we pass it around for debugging purposes, but this makes RefCtx rather ubiquitous. We might be able to remove RefCtx as an argument to internal functions in places. For example, we could:

• Also store the RefCtx in structures closer to where they are needed, and/or
• Extract RefCtx's on demand from Refs that are already in scope rather than passing in RefCtx from outside, and/or
• Bundle RefCtx with other ubiquitous arguments