RFC: Generic aliases

[stakach]

Rendered: https://github.com/stakach/rfcs/blob/rfc/generic-aliases/text/0028-generic-aliases.md

Summary

Allow alias declarations to take type parameters so a single declaration can stand in for a family of substituted types:

alias Maybe(T)        = T | Nil
alias StringKeyed(V)  = Hash(String, V)
alias Pair(K, V)      = Tuple(K, V)

A generic alias is usable everywhere a regular type is — as a restriction, in type declarations, inside generic instantiations, as a metaclass expression, and as a forall T restriction — and is purely substitutional (no new runtime type, no new dispatch).

Motivation

#2803 — today there's no way to name a parameterised shape without either copy-pasting per-variant aliases (MaybeInt32, MaybeString, …) or introducing a wrapper class with runtime cost. The RFC body covers prior art (Rust, Scala 3, TypeScript, Haskell) and the rationale for choosing transparent substitution over a nominal wrapper.

Status

Drafted at the level of user-visible semantics; intentionally avoids prescribing compiler internals so the implementation has room to land however reviewers think is cleanest. Splat parameters and parameter constraints are explicitly listed as out of scope / unresolved for follow-up RFCs.