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

(https://github.com/ruby/prism/pull/1787)

https://github.com/ruby/prism/commit/5acc38a2f3

https://github.com/ruby/prism/commit/e2e29bef2d

https://github.com/ruby/prism/commit/3193902c43

https://github.com/ruby/prism/commit/160df9a687

https://github.com/ruby/prism/commit/6d8358c083

https://github.com/ruby/prism/commit/7c9cf63d4c

Moves the common flag bits to the top. This lets us eliminate the `COMMON`
constant, and also allows us to group encoding flags on a nibble so we
can more easily mask them.

https://github.com/ruby/prism/commit/895508659e