perf(kernel/type_checker): improve infer_app peformance

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
perf(kernel/expr): minimize access to system memory allocator by recycling expr_cells

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
perf(kernel/expr): minimize access to system memory allocator by recycling expr_cells