source2il: separate `Context` creation

[zerbina]

Summary

Separate Context creation from translation (exprToIL). This is a prerequisite for incremental processing of modules.

Details

• rename Context to ModuleCtx
• the new open procedure creates a new ModuleCtx
• the new close procedures finishes the ModuleCtx and yields the produced IL code
• exprToIL appends a proc to Context and only returns the type
• Literals can now be appended to directly, without a PackedTree
• Context now accumulates its own Literals DB, instead of re-using that of the input tree(s)

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Notes For Reviewers

• part of the work of introducing modules and procedure declarations