The locals_body_index gives the index in the locals array where
the locals from the body start. This allows compilers to easily
index past the parameters in the locals array.

https://github.com/ruby/prism/commit/5d4627b890

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.

https://github.com/ruby/prism/commit/2d87303903

We are aware at parse time how many numbered parameters we have
on a BlockNode or LambdaNode, but prior to this commit, did not
store that information anywhere in its own right.

The numbered parameters were stored as locals, but this does not
distinguish them from other locals that have been set, for example
in `a { b = 1; _1 }` there is nothing on the AST that distinguishes
b from _1.

Consumers such as the compiler need to know information about how
many numbered parameters exist to set up their own tables around
parameters. Since we have this information at parse time, we should
compute it here, instead of deferring the work later on.

https://github.com/ruby/prism/commit/bf4a1e124d