Ubuntu generally does not require workarounds or tricks to get Postgres
+ PL/Rust to play nicely, so we are going to move the CI runners to use
Ubuntu 22.04 instead of AL2. This should make future updates a little
less painless.

Depends on tcdi/postgrestd#51

Previous run was a bust. This should be the correct spot.

This reverts commit 4d21a39.

This adds support for pgrx' `fn_call` interface that enables calling
other schema-defined functions dynamically.

The only code changes, as it were, are in the `plrust-trusted-pgrx`
reexport crate. The bulk of this PR is adding a little chapter to the
book along with a simple unit test.

Welcome to PL/Rust v1.2.7. This is a small feature release that adds the
ability to call user defined functions (UDFs) from a `LANGUAGE plrust`
function.

As a contrived example, perhaps your database has a `LANGUAGE sql`
function like:

```sql
CREATE OR REPLACE FUNCTION sum_array(a int[]) RETURNS int STRICT LANGUAGE sql AS $$ 
    SELECT sum(e) FROM unnest(a) e 
$$;
```

And you wish to call it from a PL/Rust function.  Well, now you can!  

```sql
CREATE OR REPLACE FUNCTION transform_array(a int[]) RETURNS int STRICT LANGUAGE plrust AS $$
    // add one to every element of `a`, the input argument array, collecting into a new Vec
    let a = a.into_iter().map(|e| e.unwrap_or(0) + 1).collect::<Vec<_>>();  

    // call the existing "sum_array(int[])" function to sum the values of `a`
    Ok(fn_call("sum_array", &[&Arg::Value(a)])?)
$$;

SELECT transform_array(ARRAY[1,2,3]);
transform_array 
-----------------
               9
(1 row)
```

PL/Rust's dynamic function call API is documented in [the
book](https://tcdi.github.io/plrust).

Other than also upgrading the underlying `pgrx` dependency to v0.11.0,
there have been no other changes to PL/Rust since v1.2.6.

This release took quite a bit longer than expected as we had a desire to
upgrade it to work with Rust v1.73.0. Unfortunately, rustc v1.73.0
introduced a bug around custom lints and it took our team *weeks* to
track this down and ultimately provide the Rust project a PR. Based on
Rust's release schedule, that fix won't be released until v1.75.0.


### A note on Postgres 16 support

Postgres 16 has added some "SIMD" code, and includes the compiler
built-in header for SIMD support. This can cause compilation problems
with pgrx if the host system has multiple `clang` versions installed.
It's suggested a machine running PL/Rust only have one `clang` version
and the matching `llvm` packages installed. It doesn't seem to matter
which version, only that there's one.