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.

ruby/prism@2d87303903