refactor to make sigmatch use LayeredIdTable for bindings

[metagn]

split from #24198

This is a required refactor for the only good solution I've been able to think of for #4858 etc. Explanation:

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sigmatch currently disables bindings (except for binding to other generic parameters) when matching against constraints of generic parameters. This is so when the constraint is a general metatype like seq, the type matching will not treat all following uses of seq as the type matched against that generic parameter.

However to solve #4858 etc we need to bind or types with a conversion match to the type they are supposed to be converted to (i.e. matching int literal(123) against int8 | int16 should bind int8¹, not int). The generic parameter constraint binding needs some way to keep track of this so that matching int literal(123) against T: int8 | int16 also binds T to int8¹.

The only good way to do this IMO is to generate a new "binding context" when matching against constraints, then binding the generic param to what the constraint was bound to in that context (in #24198 this is restricted to just or types & concrete types with convertible matches, it doesn't work in general).

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semtypinst already does something similar for bindings of generic invocations using LayeredIdTable, so LayeredIdTable is now split into its own module and used in sigmatch for type bindings as well, rather than a single-layer TypeMapping. Other modules which act on sigmatch's binding map are also updated to use this type instead.

The type is also made into an object type rather than a ref object to reduce the pointer indirection when embedding it inside TCandidate/TReplTypeVars, but only on arc/orc since there are some weird aliasing bugs on refc/markAndSweep that cause a segfault when setting a layer to its previous layer. If we want we can also just remove the conditional compilation altogether and always use ref object at the cost of some performance.

Footnotes

1. int8 binding here and not int16 might seem weird, since they match equally well. But we need to resolve the ambiguity here, in reject or types matching multiple types for int literals #24012 I tested disallowing ambiguities like this and it broke many packages that tries to match int literals to things like int16 | uint16 or int8 | int16. Instead of making these packages stop working I think it's better we resolve the ambiguity with a rule like "the earliest or branch with the best match, matches". This is the rule used in test refactor that uses LayeredIdTable in sigmatch #24198. ↩ ↩²