Split type t into Imm values and Sym hash-consed nodes

This commit fundamentally changes the representation of expressions (`t`)
by lifting constant values out of the hash-consing machinery.

Changes:

- `t` is now a variant: `Imm of Value.t | Sym of expr Hc.hash_consed`.
- Removed `Val` and `Loc` constructors from the underlying `expr` type.
- Constants are no longer hash-consed; they are stored directly as `Imm`.

Potential Performance Improvements:

- Creating constant expressions  no longer requires a hash-cons table lookup or allocation of a hash-cons node.
- The global hash-cons table is no longer polluted with ephemeral constant values.

BREAKING CHANGE: Previously, because all expressions were hash-consed, structural
equality implied physical equality (`phys_equal` was sufficient for all comparisons).

This invariant no longer holds for `Imm` values.

Author: filipeom

src/smtml/dune

@@ -29,7 +29,6 @@
   interpret
   interpret_intf
   lexer
-  loc
   log
   logic
   mappings