mir: newtype peeling

[sbillig]

This (recursively) replaces single-field structs with their field, while retaining the original type information from the. Result is many fewer memory allocations, and free "newtype" wrapper structs like struct Wei { inner: u256 }.

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[sbillig]

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Pull request overview

This PR implements "newtype peeling" in the MIR (Mid-level Intermediate Representation) layer, which recursively replaces single-field structs with their underlying field values while preserving type information. The optimization reduces memory allocations by treating transparent newtype wrappers (like struct Wei { inner: u256 }) as their inner type at runtime.

Key changes include:

• A new repr.rs module centralizing runtime representation queries with recursive newtype unwrapping
• A canonicalization transform pass (canonicalize_transparent_newtypes) that rewrites transparent-newtype projections
• Extension of StorageMap to include a const SALT parameter for type-level discrimination between storage maps
• Updates to expression lowering, match lowering, and effect argument handling to support transparent cast operations

Reviewed changes

Copilot reviewed 57 out of 57 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
crates/mir/src/repr.rs	New module defining canonical representation categories with transparent newtype peeling logic
crates/mir/src/transform.rs	Adds canonicalize_transparent_newtypes pass to flatten transparent cast chains and rewrite place projections
crates/mir/src/lower/mod.rs	Integrates new canonicalization pass and refactors representation queries to use centralized logic
crates/mir/src/lower/prepass.rs	Extends const expression handling to support const generic parameters in type positions
crates/mir/src/lower/expr.rs	Major refactoring of expression lowering to handle transparent casts, effect arguments, and assignment to transparent newtype fields
crates/mir/src/lower/match_lowering.rs	Adds transparent cast handling for non-reference scrutinee projections in match expressions
crates/mir/src/lower/aggregates.rs	Simplifies record literal lowering for transparent newtypes
crates/mir/src/monomorphize.rs	Adds const type parameter substitution in monomorphization
crates/mir/src/ir.rs	Extracts try_value_address_space_in helper for reuse in transform pass
library/std/src/evm/storage_map.fe	Breaks API: adds required const SALT parameter to StorageMap for collision prevention
Test fixtures	Updates MIR/codegen snapshots and adds new test cases for newtype scenarios

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Codex Review: Didn't find any major issues. Another round soon, please!

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