Lots of changes. Evaluations, not expressions.

[katzdm]

Also:

• value_of -> extract
• reflect_value -> reflect
• new value_of, is_evaluation metafunctions
• tried to update all uses of "constant value" or "value" to "evaluation" (where appropriate)
• removed support for reflecting cast-expressions
• removed some details of historical discussion (e.g., alternative syntaxes) to avoid further bikeshedding
• editing pass over the entire paper (excluding library wording).