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

Fundamentally, `foo { |bar,| }` is different from `foo { |bar, *| }`
because of arity checks. This PR introduces a new node to handle
that, `ImplicitRestNode`, which goes in the `rest` slot of parameter
nodes instead of `RestParameterNode` instances.

This is also used in a couple of other places, namely:

* pattern matching: `foo in [bar,]`
* multi target: `for foo, in bar do end`
* multi write: `foo, = bar`

Now the only splat nodes with a `NULL` value are when you're
forwarding, as in: `def foo(*) = bar(*)`.

https://github.com/ruby/prism/commit/dba2a3b652

https://github.com/ruby/prism/commit/b7850f2d30

https://github.com/ruby/prism/commit/f0aa8ad93b

https://github.com/ruby/prism/commit/c2b7724d91

https://github.com/ruby/prism/commit/c31f61e898

Prior to this commit, KeywordParameterNode included both optional
and required keywords. With this commit, it is split in two, with
`OptionalKeywordParameterNode`s no longer having a value field.

https://github.com/ruby/prism/commit/89084d9af4

https://github.com/ruby/prism/commit/e6deed05a5

https://github.com/ruby/prism/commit/6d1858192e