compat with ReScript v10

[zth]

Fixes #373

The issue was that the ReScript v10 parser inserts (internal) attributes at positions that ppxlib does not consider valid OCaml.

Solution short/mid term: use forked ppxlib that has been patched to ignore the invalid attributes.
Solution long term: Move off of ppxlib (and other OCaml-specific libraries completely where ReScript might break compat).

• More relevant background information on the solution: Be more relaxed about ppx attributes. janestreet/ppx_inline_test#15
• The fork of ppxlib with the fix applied: https://github.com/zth/ppxlib/tree/rescript-attrs