feat: arena-based Copy expressions for BaseExpr and ExtExpr

[ClementWalter]

Summary

• Replace Box<T> recursive variants with arena allocation using u32 indices
• Makes BaseExpr and ExtExpr Copy types, eliminating the need to manually .clone() every expression reuse in constraint definitions
• String interning for Param variants (StringId instead of String)

Motivation

When building AIR constraints, every reuse of an expression required manual .clone() calls:

// Before (tedious and error-prone)
let enabler = cols.opcode_add_flag.clone()
    + cols.opcode_sub_flag.clone()
    + cols.opcode_xor_flag.clone();

// After (expressions are Copy)
let enabler = cols.opcode_add_flag
    + cols.opcode_sub_flag
    + cols.opcode_xor_flag;

Implementation

• New arena.rs module with ExprArena, thread-local storage, and node types
• BaseExpr(u32) and ExtExpr(u32) are now just indices into the arena
• All operator impls use thread-local arena for allocation
• Pattern matching via .node() method instead of direct enum access

Benefits

• No more manual .clone() calls on expression reuse
• Better cache locality from contiguous arena storage
• Potential performance improvement from reduced allocations

Test plan

• All existing tests pass with --features prover
• Benchmark constraint evaluation performance

[reviewable-StarkWare]

This change is