Make a function's locally abstract types part of the function node

[cristianoc]

Part of the n-ary functions series tracked in #8557 (item 8 of 10; items 1–7 are merged).

What

Replace the Pexp_newtype wrapper chains that the parser built for
(type t, x) => ... arrow syntax with a structural field on the function
node: Pexp_fun.newtypes carries each newtype name with its own
attributes, hoisted in front of the value parameters as before.
Pexp_newtype remains solely as the desugaring of [let f: type a. ...]
annotations and for PPX-authored trees.

Fidelity fixes visible in the formatter:

• Attributes keep their association with their type parameter group:
  (@attr type t, x, @attr2 type s, y) round-trips as written instead of
  printing @attr @attr2 on the function.
• Comments written next to a type parameter travel with it to the
  hoisted group instead of migrating onto the following value parameter.
• Attributes written in front of the arrow now live on the function
  node, so built-in attribute processing (e.g. @this) sees them on
  type-first functions; previously they sat inert on the wrapper node.

Typing follows the upstream OCaml 5.x design: the newtype machinery is
extracted into a reusable type_newtype helper (mirroring OCaml's helper
of the same name) and the function case peels one newtype at a time,
mimicking the typing of the former wrapper chain; the typedtree output
is bit-identical to before.

The v0 PPX bridge expands the field back into a wrapper chain around
Function$: each wrapper carries its own newtype's attributes, and the
outermost wrapper separates function-node attributes from the first
newtype's attributes with an internal _res.newtype_attrs marker (no
marker means node attributes only, matching the historical wire).
Newtype-free programs are wire byte-identical; for functions with
newtypes the deltas are confined to wrapper-node locations and, for the
rare attributed groups, per-wrapper attribute placement. Identity-PPX
round-trips are AST-exact, verified against the previous compiler.

Also: jsx_v4 and bs_builtin_ppx now carry newtypes (and their
attributes) through their function rebuilds instead of dropping them,
the sexp AST debugger emits the field, and dead parser plumbing
(fundef param attrs/p_pos, arrow_start_pos, make_newtypes ~attrs) is
removed.

🤖 Generated with Claude Code

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[codecov]

Codecov Report

❌ Patch coverage is 83.67347% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.85%. Comparing base (36b14d6) to head (1e39b9f).

Files with missing lines	Patch %	Lines
compiler/ml/ast_mapper_from0.ml	70.00%	6 Missing ⚠️
compiler/ml/printast.ml	0.00%	4 Missing ⚠️
compiler/frontend/bs_ast_mapper.ml	0.00%	3 Missing ⚠️
compiler/syntax/src/res_core.ml	88.46%	3 Missing ⚠️
compiler/ml/typecore.ml	92.59%	2 Missing ⚠️
compiler/syntax/src/res_comments_table.ml	75.00%	2 Missing ⚠️
compiler/syntax/src/res_parsetree_viewer.ml	83.33%	2 Missing ⚠️
compiler/frontend/bs_builtin_ppx.ml	75.00%	1 Missing ⚠️
tools/src/transforms.ml	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8574      +/-   ##
==========================================
- Coverage   75.89%   75.85%   -0.04%     
==========================================
  Files         474      474              
  Lines       62772    62835      +63     
==========================================
+ Hits        47640    47665      +25     
- Misses      15132    15170      +38     

Files with missing lines	Coverage Δ
compiler/ml/ast_helper.ml	78.73% <100.00%> (ø)
compiler/ml/ast_iterator.ml	92.78% <100.00%> (-0.28%)	⬇️
compiler/ml/ast_mapper.ml	75.46% <100.00%> (+0.11%)	⬆️
compiler/ml/ast_mapper_to0.ml	58.45% <100.00%> (+0.87%)	⬆️
compiler/ml/pprintast.ml	91.56% <100.00%> (-0.31%)	⬇️
compiler/syntax/src/jsx_v4.ml	86.70% <100.00%> (-0.03%)	⬇️
compiler/syntax/src/res_ast_debugger.ml	96.14% <100.00%> (+0.02%)	⬆️
compiler/syntax/src/res_printer.ml	93.18% <100.00%> (+<0.01%)	⬆️
compiler/frontend/bs_builtin_ppx.ml	89.18% <75.00%> (-1.44%)	⬇️
tools/src/transforms.ml	0.00% <0.00%> (ø)
... and 7 more

... and 1 file with indirect coverage changes

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