The current spanZ() function will not scan for a 0 terminator if the
type is not 0 terminated. This encourages using 0 terminated array
types to bind C arrays which hold 0 terminated strings. However, this is
a big footgun as nothing in the C type system guarantees there to be a
0 terminator at the end of the array and if there is none this becomes
Illegal Behavior in Zig.
To solve this, deprecate spanZ() and lenZ(), adding a new sliceTo()
function that always scans for the given terminator even if the type is
not sentinel terminated.
cd7b5a3718