transpile: Create new context.rs module and split ExprContext into multiple types

[Rua]

• Depends on transpile: Refactor, document and correct uses of used() flag #1975

Some context flags don't apply to all possible child nodes, so the split avoids accidentally passing/inheriting them when not needed. That helps deal with #1973 (comment), though there are further changes that are needed after this, to "reset" the ExprContext specific flags for each subexpression.

The reason that needs_address got renamed is mainly for easier naming of the new getter functions. If getters for booleans are to be named is_*, then is_needs_address doesn't work very well, while is_address_needed works much better. The decay_ref flag was left public because it's going to be removed by #1982 anyway, so there's no point in investing extra time thinking of a way to make it fit the rest of the refactor.