Rename "local variables" to "explicit stack slots".

The term "local variables" predated the SSA builder in the front-end
crate, which also provides a way to implement source-language local
variables. The name "explicit stack slot" makes it clear what this
construct is.

lib/reader/src/parser.rs

@@ -1044,7 +1044,7 @@ impl<'a> Parser<'a> {
     // Parse a stack slot decl.
     //
     // stack-slot-decl ::= * StackSlot(ss) "=" stack-slot-kind Bytes {"," stack-slot-flag}
-    // stack-slot-kind ::= "local"
+    // stack-slot-kind ::= "explicit_slot"
     //                   | "spill_slot"
     //                   | "incoming_arg"
     //                   | "outgoing_arg"