Binary Operators (and Grouping in Expressions)

Add support for <, <=, >, >=, && and ||.

Also, add support for grouping (with ( and )) in expressions.

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

tree-sitter-p4/test/corpus/declarations.txt

@@ -169,7 +169,9 @@ parser simple() {
         (identifier)
         (assignment)
         (expression
-         (string_literal)
+         (simple_expression
+          (string_literal)
+         )
         )
        )
       )