Rollup merge of #109179 - llogiq:intrinsically-option-as-slice, r=eholk

move Option::as_slice to intrinsic

````@scottmcm```` suggested on #109095 I use a direct approach of unpacking the operation in MIR lowering, so here's the implementation.

cc ````@nikic```` as this should hopefully unblock #107224 (though perhaps other changes to the prior implementation, which I left for bootstrapping, are needed).

Author: Dylan-DPC