Add ability to mark code generated for some domains to be Kotlin-internal

[dlurton]

Currently, multiple permuted domains may exist and the generated code is entirely Kotlin public, however: the client package may wish to expose a subset of those domains as public API, or none at all.

For instance, in partiql-lang-kotlin, we might have several permuted domains:

partiql_ast -> partiql_ast_var_decl -> partiql_algebra_unindexed -> partiql_algebra -> partiql_physical

Of those, the only ones we might wish to expose as public API are: partiq_ast, partiql_algebra and partiql_physical.

partiql-lang-kotlin will need a way to mark only some of these domains as public.

Potentially, we could add syntax to the domain definition:

(define public partiql_ast (domain ...)

(define internal partiql_ast_var_decl (permute_domain ...))

This would be backward-incompatible with previous versions, but this might be unavoidable, unless we want to the default visibility to public. This is sub-optimal because users could accidentally expose generated code publicly that they did not intend.

Either:

• The default should be internal
• The visibility must be specified with every define.