tamer: parse::ParseState::Context: Remove Default trait bound

This is too restrictive, especially for parsers that fold into something,
like the ASG, which may exist prior to invoking the parser.

This moves the trait bound to the functions that actually need it.  Those
obviously cannot be used if the Context does not implement `Default`, but
I'll provide alternative conveniences.

DEV-11864

tamer/src/parse.rs

@@ -123,13 +123,16 @@ pub trait ParseState: Default + PartialEq + Eq + Debug {
     /// Errors specific to this set of states.
     type Error: Debug + Diagnostic + PartialEq + Eq;
 
-    type Context: Default + Debug = EmptyContext;
+    type Context: Debug = EmptyContext;
 
     /// Construct a parser.
     ///
     /// Whether this method is helpful or provides any clarity depends on
     ///   the context and the types that are able to be inferred.
-    fn parse<I: TokenStream<Self::Token>>(toks: I) -> Parser<Self, I> {
+    fn parse<I: TokenStream<Self::Token>>(toks: I) -> Parser<Self, I>
+    where
+        Self::Context: Default,
+    {
         Parser::from(toks)
     }
 
@@ -681,6 +684,7 @@ impl<S: ParseState, I: TokenStream<S::Token>> Parser<S, I> {
     where
         LS: ParseState<Token = S::Object>,
         <S as ParseState>::Object: Token,
+        <LS as ParseState>::Context: Default,
     {
         self.while_ok(|toks| {
             // TODO: This parser is not accessible after error recovery!
@@ -883,7 +887,12 @@ impl<T: Token, E: Diagnostic + PartialEq + Eq + 'static> Diagnostic
     }
 }
 
-impl<S: ParseState, I: TokenStream<S::Token>> From<I> for Parser<S, I> {
+impl<S, I> From<I> for Parser<S, I>
+where
+    S: ParseState,
+    I: TokenStream<S::Token>,
+    <S as ParseState>::Context: Default,
+{
     fn from(toks: I) -> Self {
         Self {
             toks,