lang0: split Conv into multiple operators

[zerbina]

Summary

Add the Zext, Sext, Trunc, Promote, and Demote operators.
Conv still exists, but is now only used for int-to-float and float-
to-int conversions.

Details

The previous Conv operator was modeled after the conversion operator
of NimSkull's MIR, making it fairly high-level and leaving too much
behaviour-related decision making to pass0.

It's split into multiple, lower-level operators, making the final
translation easier and also providing earlier transformation stages with
more precise conversion operators.

skully is changed to:

• use the new operators instead of Conv (where applicable)
• translate pointer-to-pointer conversions to IL no ops (as all
  pointers are translated to the same IL type)

Finally, tests for the new L0 operators are added, except for Conv,
whose semantics aren't very well-defined at the moment.

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Notes For Reviewers

A preparation for #146, but the new operators should be helpful in general, especially once the source language starts to support numeric conversions.

[saem]

I personally prefer source then destination (reading things as "from to") as the order, but if there is a good reason not to do that, I get it.

[zerbina]

I personally prefer source then destination (reading things as "from to") as the order, but if there is a good reason not to do that, I get it.

I chose the current order for reasons of regularity.

For most other operators that return a value (e.g., Add, Sub, BitAnd, etc.), the type of the result comes as the first operand (they don't have a second type operand), so when designing the initial Conv operator, I opted for having the destination type come first, so that child(n, 0) for a Conv tree also returns the type operand representing the type of the operation.

There are some exceptions to the "first type operand is result type" rule, however, namely the comparison operators, Not, and Call.