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

Ruby allows for 0 or negative line start, this is often used
with `eval` calls to get a correct offset when prefixing a snippet.

e.g.

```ruby
caller = caller_locations(1, 1).first
class_eval <<~RUBY, caller.path, caller.line - 2
  # frozen_string_literal: true
  def some_method
    #{caller_provided_code_snippet}
  end
RUBY
```

https://github.com/ruby/prism/commit/0d14ed1452

https://github.com/ruby/prism/commit/33cc75a4b7

It's possible for us to parse a constant that is invalid in the
current encoding. To fix this, we wrap the intern function in an
rb_protect to ensure we always put a valid symbol into the AST.

https://github.com/ruby/prism/commit/e78398554d

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

https://github.com/ruby/prism/commit/6f243de0c7