Support scannable axes

[julesjacobs]

These now parse and format correctly:

type t : any non_pointer
type t : value non_pointer
type t : value maybe_pointer
type t : value maybe_pointer non_pointer

Qualified abbreviations are supported too:

type t : Foo.bar non_pointer

And scannable modifiers compose correctly with existing kind syntax:

type t : value non_pointer with type_
type t : value non_pointer mod mode

These are preserved rather than rewritten:

type t : Foo.immediate
type t : immediate non_pointer
type t : Foo.immediate64
type t : immediate64 maybe_pointer

Note:

• The parser now matches OxCaml directly by accepting type_longident as the abbreviation head, rather than narrower
• The formatter preserves scannable modifiers in source order instead of normalizing them (do we want to normalize?)
• The immediate / immediate64 rewrite path now only rewrites unqualified & modifier-free abbreviations, to avoid removing scannable-axis components

[mdelvecchio-jsc]

Add a test with comments between the layout and the scannable axes, e.g. type t : value (* hi *) non_pointer. Then should be good.

[julesjacobs]

Thanks Megan! I've added the test