fix: inferStatePrefix... should default to s1 as the first interm…

…ediate state. (#199)

### Description:

We claim in the error that we fall back to `s1` as the first
intermediate state if we can't infer anything.
However, the implementation actually used `s2`. 

This PR fixes that by setting `currentStateNumber := 0` in the fallback
case. The off-by-one behaviour is because this number is incremented
*before* it's used to generate a new state.

### Testing:

What tests have been run? Did `make all` succeed for your changes? Was
conformance testing successful on an Aarch64 machine? Yes

### License:

By submitting this pull request, I confirm that my contribution is
made under the terms of the Apache 2.0 license.

Tactics/SymContext.lean

@@ -247,7 +247,7 @@ Falling back to the default numbering scheme, \
 with `s1` as the first new intermediate state"
     modifyThe SymContext ({ · with
       state_prefix := "s",
-      currentStateNumber := 1 })
+      currentStateNumber := 0 })
 
 /-- Annotate any errors thrown by `k` with a local variable (and its type) -/
 private def withErrorContext (name : Name) (type? : Option Expr) (k : MetaM α) :