the default is now MAX priority

We are going to start enforcing the invariant that an expression
with priority N can only directly things of higher priority.
In that case for something like this:

```
enum Expr {
    Id(Id),
    Literal(Literal),

    #[precedence(1, left)]
    Add(Expr, Expr),

    #[precedence(2, left)]
    Mul(Expr, Expr),
}
```

you want Id/Literal to be max priority so they can be embedded
into anything.

Author: nikomatsakis

crates/formality-core/src/parse/parser.rs

@@ -107,7 +107,7 @@ impl Precedence {
 
 impl Default for Precedence {
     fn default() -> Self {
-        Self::new(0, Associativity::None)
+        Self::new(std::usize::MAX, Associativity::None)
     }
 }