Add checks for overflow

[dsharlet]

We should add error checking for overflow in slinky pipeline evaluation.

Adding checked arithmetic on the expression evaluation and buffer operations (e.g. for_each_element) could be expensive due to adding error handling codepaths.

Instead, I think we can ensure that there is no overflow when constructing buffers, and when calling pipelines. We can:

1. Add a validate_buffer(const raw_buffer&) function that verifies that both the min and max address accessible by the buffer do not overflow. This will probably be similar to this function:

   slinky/slinky/runtime/buffer.cc

   Lines 26 to 38 in e5ef18c

   	std::size_t alloc_size(std::size_t rank, std::size_t elem_size, const dim* dims) {
   	index_t flat_min = 0;
   	index_t flat_max = 0;
   	for (std::size_t i = 0; i < rank; ++i) {
   	if (dims[i].stride() == 0) continue;
   	index_t extent = alloc_extent(dims[i]);
   	assert(extent >= 0);
   	if (extent == 0) return 0;
   	flat_min += (extent - 1) * std::min<index_t>(0, dims[i].stride());
   	flat_max += (extent - 1) * std::max<index_t>(0, dims[i].stride());
   	}
   	return flat_max - flat_min + elem_size;
   	}

   .
2. Call validate_buffer in evalaute for make_buffer and allocate. These evaluators can return an error code. I think that all other buffer operations cannot cause overflow to occur.
3. Add a call expression for validate_buffer to

   slinky/slinky/runtime/expr.h

   Line 87 in e5ef18c

   enum class intrinsic {

   and then generate checks for validate_buffer when constructing pipelines (probably here:

   slinky/slinky/builder/pipeline.cc

   Line 1446 in e5ef18c

   void check_buffer(const buffer_expr_ptr& b, std::vector<stmt>& checks) {

   )

We should test this in https://github.com/dsharlet/slinky/blob/main/slinky/builder/test/checks.cc (at least).