Add missing parentheses in type expr (_ as 'a) t

[Julow]

This bug is found for example in Eio: https://ocaml.org/p/eio/latest/doc/Eio/Domain_manager/index.html#type-t
Type t renders as invalid syntax:

type 'a t = [> ty ] as 'a Resource.t

In the source code, it's:

type 'a t = ([> ty] as 'a) Resource.t

[jonludlam]

Looks good, thanks @Julow

The test failure is due to the mpopcnt bug on ARM - a consequence of needing base which I think is only used for Sherlodoc's test suite. It'd be good to fix this but for now I'm just going to ignore the failure.

[jonludlam]

Oops, forgot the CHANGES entry!