Support struct values in storage maps

[g-r-a-n-t]

Adds typed pointers to StorageMap entries, allowing maps to store and access multi-word values such as structs without requiring those values to implement WordRepr.

Pointer-based access uses the normal storage effect model. Existing get and set APIs remain available for single-word WordRepr values.

API

• Adds StorageMap::ptr, returning a read-only StoragePtr<V>.
• Adds StorageMap::mut_ptr, returning a mutable StorageMutPtr<V>.
• Requires mutable access to the map before creating a mutable entry pointer.
• Implements the standard effect-handle and effect-reference traits for these pointers.
• Keeps StorageMap::get and StorageMap::set restricted to V: WordRepr.

Storage layout

The map entry hash is treated as the base storage slot for the value. Aggregate fields are projected relative to that slot using the compiler's storage layout.

This PR also defines consistent packed storage layout for nominal structs:

• Adjacent scalar fields smaller than one word may share a storage slot.
• Writes to shared slots use read-modify-write operations so neighboring fields are preserved.
• Nested structs use their recursively computed packed storage span.
• Tuples, arrays, and other aggregate transitions remain storage boundaries.
• Signed and narrow scalar values are stored in their canonical bit representation.

HIR layout reporting and MIR/codegen now use the same shared field-layout planner. The same rules therefore apply to both ordinary contract struct fields and structs reached through storage-map entry pointers. Memory and ABI layouts are unchanged.

Example

struct Position {
  owner: u256,
  collateral: u256,
  debt: u256,
}

struct Store {
   positions: StorageMap<u256, Position>,
}

let position = store.positions.ptr(key: account)
let mut position = store.positions.mut_ptr(key: account)

The read-only pointer can inspect the entry, while the mutable pointer can replace the value or update individual fields through normal storage-provider semantics.

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