add workspace_hack crate

Our builds can be a little inconsistent, because Cargo doesn't deal well
with workspaces where there are multiple crates which have different
dependencies that select different features. As a workaround, copy what
other big rust projects do: add a workspace_hack crate.

This crate just pins down a set of dependencies and features that
satisfies all of the workspace crates.

The benefits are:
- running `cargo build` from one of the workspace subdirectories now
  works without rebuilding anything.
- running `cargo install` works (without rebuilding anything).
- making small dependency changes is much less likely to trigger large
  dependency rebuilds.

postgres_ffi/Cargo.toml

@@ -17,6 +17,7 @@ crc32c = "0.6.0"
 hex = "0.4.3"
 log = "0.4.14"
 thiserror = "1.0"
+workspace_hack = { path = "../workspace_hack" }
 
 [build-dependencies]
 bindgen = "0.57"