Remove public hidden symbol function

Now, you can't a particular occurrence of a symbol
in a grammar. You can only hide a symbol globally
(right now, by beginning its name with an underscore).

src/compiler/rules/rules.cc

@@ -37,10 +37,6 @@ namespace tree_sitter {
             return make_shared<Symbol>(name);
         }
 
-        rule_ptr _sym(const string &name) {
-            return make_shared<Symbol>(name, SymbolTypeHidden);
-        }
-
         rule_ptr pattern(const string &value) {
             return make_shared<Pattern>(value);
         }

src/compiler/rules/symbol.cc

@@ -32,8 +32,6 @@ namespace tree_sitter  {
             switch (type) {
                 case SymbolTypeNormal:
                     return string("#<sym '") + name + "'>";
-                case SymbolTypeHidden:
-                    return string("#<hidden_sym '") + name + "'>";
                 case SymbolTypeAuxiliary:
                     return string("#<aux_sym '") + name + "'>";
                 case SymbolTypeBuiltIn:
@@ -56,7 +54,7 @@ namespace tree_sitter  {
         }
 
         bool Symbol::is_hidden() const {
-            return (type == SymbolTypeHidden || type == SymbolTypeAuxiliary);
+            return (name.front() == '_' || type == SymbolTypeAuxiliary);
         }
 
         void Symbol::accept(Visitor *visitor) const {

src/compiler/rules/symbol.h

@@ -9,7 +9,6 @@ namespace tree_sitter  {
     namespace rules {
         typedef enum {
             SymbolTypeNormal,
-            SymbolTypeHidden,
             SymbolTypeAuxiliary,
             SymbolTypeBuiltIn
         } SymbolType;