Include emergency stack slots when laying out the stack.

Emergency stack slots are a new kind of stack slot added relatively
recently. They need to be allocated a stack offset just like explicit
and spill slots.

Also, make StackSlotData's offset field an Option, to catch problems
like this in the future. Previously the value 0 was used when offsets
weren't assigned yet, however that made it non-obvious when the field
meant "not assigned yet" and when it meant "assigned the value 0".

lib/reader/src/parser.rs

@@ -1070,7 +1070,7 @@ impl<'a> Parser<'a> {
         // Take additional options.
         while self.optional(Token::Comma) {
             match self.match_any_identifier("expected stack slot flags")? {
-                "offset" => data.offset = self.match_imm32("expected byte offset")?,
+                "offset" => data.offset = Some(self.match_imm32("expected byte offset")?),
                 other => return err!(self.loc, "Unknown stack slot flag '{}'", other),
             }
         }