Add inventory-0-3-1 optional feature

[danielhenrymantilla]

1. In more recent rust toolchains and/or specific platforms, the ctor hack which inventory relied on was broken (causing safer_ffi may randomly generate empty headers #131);
2. Since Rust 1.62.0, inventory 0.3.* has been released which, would solve the issue.
3. But the MSRV of safer-ffi is 1.60.0.

Thence the idea to rely on a Cargo feature for 1.62.0+ users to replace inventory with inventory 0.3.1. This, thus, could have been expected to fix the issue #131 (EDIT: it doesn't 😔).

• @pan93412 can you try this and confirm that it fixes the issue?

  [patch.crates-io.safer-ffi]
  git = "https://github.com/getditto/safer_ffi.git"
  branch = "inventory-bump"

  And then, at https://github.com/pan93412/libstock-ffi/blob/e87b04cf6e1bd9394db656315f1dbd2975418c95/Cargo.toml#L27, you should add inventory-0-3-1 as well:

  - headers = ["safer-ffi/headers"]
  + headers = ["safer-ffi/headers", "safer-ffi/inventory-0-3-1"]

[pan93412]

Unfortunately, it is still not working 😞

However, I found that some parts of safer-ffi still depended on inventory v0.1.11.

But header::generate seems called inventory 0.3.1, as expected:

I have no idea for why did this happen after upgrading. Maybe we should report it to the upstream? Not sure about it.

[pan93412]

head is still null. I can let you connect to my machine to debug for it. The new commit is wmjtyd/libstock-ffi@cb57d4c

[danielhenrymantilla]

I'll follow up the discussion on the original issue, since inventory 0.3.1 is not a solution

[pan93412]

Although Inventory 0.3.1 did not fix up that issue, could we can merge it into master for some people who prefer the newer Inventory version?