* resolve_inferred_alloc now gives a proper mutability attribute to the
   corresponding alloc instruction. Previously, it would fail to mark
   things const.
 * slicing: fix the detection for when the end index equals the length
   of the underlying object. Previously it was using `end - start` but
   it should just use the end index directly. It also takes into account
   when slicing a comptime-known slice.
 * `Type.sentinel`: fix not handling all slice tags