[ruby/prism] Change numbered parameters

Previously numbered parameters were a field on blocks and lambdas
that indicated the maximum number of numbered parameters in either
the block or lambda, respectively. However they also had a
parameters field that would always be nil in these cases.

This changes it so that we introduce a NumberedParametersNode that
goes in place of parameters, which has a single uint8_t maximum
field on it. That field contains the maximum numbered parameter in
either the block or lambda.

As a part of the PR, I'm introducing a new UInt8Field type that
can be used on nodes, which is just to make it a little more
explicit what the maximum values can be (the maximum is actually 9,
since it only goes up to _9). Plus we can do a couple of nice
things in serialization like just read a single byte.

2d87303903

lib/prism/debug.rb

@@ -103,9 +103,14 @@ module Prism
         case node
         when BlockNode, DefNode, LambdaNode
           names = node.locals
-
-          params = node.parameters
-          params = params&.parameters unless node.is_a?(DefNode)
+          params =
+            if node.is_a?(DefNode)
+              node.parameters
+            elsif node.parameters.is_a?(NumberedParametersNode)
+              nil
+            else
+              node.parameters&.parameters
+            end
 
           # prism places parameters in the same order that they appear in the
           # source. CRuby places them in the order that they need to appear