mpk: add an example testing the memory limits

[abrown]

This change adds an example program showing:

• how to build a pool-allocated engine that uses MPK
• the difference in the number of memory slots when creating an engine with MPK versus not

The output could look like:

$ cargo run --example mpk
without MPK:    14591 memory slots      85.5 TiB reserved
with MPK:       218861 memory slots     85.5 TiB reserved
                14.999x more slots per reserved memory

The example program probes exponentially larger memory pools to find the maximum available on the current system. This search is highly dependent on the memory available, the OS limits on VMAs, and various engine configuration parameters. See the example docs for more details.

[abrown]

cargo vet --locked should fail here with output like:

$ cargo vet --locked
Vetting Failed!

14 unvetted dependencies:
  bindgen:0.68.1 missing ["safe-to-run"]
  bytesize:1.3.0 missing ["safe-to-run"]
  cexpr:0.6.0 missing ["safe-to-run"]
  clang-sys:1.6.1 missing ["safe-to-run"]
  lazycell:1.3.0 missing ["safe-to-run"]
  libproc:0.14.2 missing ["safe-to-run"]
  mach2:0.4.1 missing ["safe-to-run"]
  minimal-lexical:0.2.1 missing ["safe-to-run"]
  nom:7.1.3 missing ["safe-to-run"]
  peeking_take_while:0.1.2 missing ["safe-to-run"]
  prettyplease:0.2.15 missing ["safe-to-run"]
  proc-maps:0.3.2 missing ["safe-to-run"]
  shlex:1.2.0 missing ["safe-to-run"]
  syn:2.0.32 missing ["safe-to-deploy"]

How do we want to handle this? The two added dependencies are dev-dependencies; should they require a vet? And if so, which kind?

[alexcrichton]

The bytesize crate looks vet-able relatively easily but looks like proc-maps is a pretty heavyweight dependency pulling in quite a lot. Could that dependency be dropped in favor of parsing /proc/self/maps manually?

[alexcrichton]

Neat! I like having little example programs like this.

[github-actions]

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