Proto/Michelson: Extract unparsing functions from Script_typed_ir.

Context

Another step towards #1374.

This MR extracts unparsing functions from Script_ir_translator into their own module Script_ir_unparser. Unfortunately the most important function in the latter module, parse_data depends on a couple of functions from Script_ir_translator. For the moment the dependency is broken using a functor, whose output is included (and re-exported from) Script_ir_translator. This could perhaps be resolved in a better way, but for the moment I have no good idea how to do it. Alternatively, functions from the functor could be moved to yet another module, depending on Script_ir_translator (in which case the functor wouldn't be needed anymore).

There might also be performance concerns if the functor isn't inlined during compilation (is there a way to force this?).

As a bonus, some docstrings are added to the interface of Script_ir_unparsing.

Manually testing the MR

There are no semantic changes, only code being moved around, so type-checking + CI.

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