Merge 'Introduce instruction VTABLE' from Lâm Hoàng Phúc

this PR improves 3-6% for `prepare` benchmark without slowing down
others.  After this PR we don't have to store `InsnFunction`  in
`Program` and `ProgramBuilder` anymore, because `to_function` will
return result without matching.

Reviewed-by: Preston Thorpe <preston@turso.tech>

Closes #3098

core/incremental/expr_compiler.rs

@@ -411,7 +411,8 @@ impl CompiledExpression {
                 // Execute the program
                 let mut pc = 0usize;
                 while pc < program.insns.len() {
-                    let (insn, insn_fn) = &program.insns[pc];
+                    let (insn, _) = &program.insns[pc];
+                    let insn_fn = insn.to_function();
                     state.pc = pc as u32;
 
                     // Execute the instruction