Add struct `uv__loop_internal_fields_t` as a location for future
additions to `uv_loop_t` while also maintaining v1.x compatibility.
Currently `uv__loop_internal_fields_t` only contains the `flags` field.
The reason for adding the `flags` field is because the same field was
never added to `UV_LOOP_PRIVATE_FIELDS` in Windows as it was in Unix.
The idea for creating a struct and attaching it to `uv_loop_t` for
future API enhancements was taken from a comment by bnoordhuis in:
https://github.com/libuv/libuv/issues/2506#issuecomment-540050665
Also add `internal_fields` to `uv_loop_t` to store the pointer to
`uv__loop_internal_fields_t`. This naming makes more sense than just
using `active_reqs.unused[1]`. To maintain ABI compatibility, shrink the
`unused` array.
PR-URL: https://github.com/libuv/libuv/pull/2725
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Jameson Nash <vtjnash@gmail.com>
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