Add an RFC for Const from shape-castable.

text/0042-const-from-shape-castable.md

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+- Start Date: (fill me in with today's date, YYYY-MM-DD)
+- RFC PR: [amaranth-lang/rfcs#42](https://github.com/amaranth-lang/rfcs/pull/42)
+- Amaranth Issue: [amaranth-lang/amaranth#0000](https://github.com/amaranth-lang/amaranth/issues/0000)
+
+# `Const` from shape-castable
+
+## Summary
+[summary]: #summary
+
+Allow passing a shape-castable to `Const`.
+
+## Motivation
+[motivation]: #motivation
+
+We currently have two incompatible syntaxes for making a constant, depending on whether it's made from a `Shape` or a shape-castable.
+The former uses `Const(value, shape)`, while the latter requires `shape.const(value)`.
+
+Making `Const` accept shape-castables means we'll have a syntax that works for all shape-likes, reducing the need to special case for shape-castables.
+
+## Guide- and reference-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+`Const(value, shape)` checks whether `shape` is a shape-castable and returns `shape.const(value)` when this is the case.
+
+## Drawbacks
+[drawbacks]: #drawbacks
+
+- A `Const()` constructor sometimes returning non-`Const` objects can be confusing.
+  - `Signal()` already behaves this way.
+
+## Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+- This is consistent with how `Signal()` handles shape-castables.
+
+Alternatives:
+- Do not do this. Require code that makes constants from a passed shape-like to check whether it got passed a shape-castable or not and pick the appropriate syntax.
+
+## Prior art
+[prior-art]: #prior-art
+
+[RFC #15](0015-lifting-shape-castables.md) added the equivalent behavior to `Signal()`.
+
+## Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+None.
+
+## Future possibilities
+[future-possibilities]: #future-possibilities
+
+None.