Keep `(context, expression)` in `Ast_aggregated` for test purging

type:internal

For LIGO developers

This MR comes from discussion with Rémi.

There's a new pass that does test purging (i.e. removing test related definitions), see !2010 (merged). However, this pass depends on the form of an aggregated contract in Ast_aggregated: a let a1 = ... in let a2 = ... in main.

This MR keeps context (sequence of defs. a1, a2, ...) separated from main expression until a later stage, giving Self_ast_aggregated access to the context + expression to some passes, which can profit from this presentation.

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