🏗️ Add examples directory

[jharrilim]

This PR adds some new examples to the repo:

1. A simple example, akin to the one found on the readme
2. An example of how to use the connection pool feature
3. An example of using the connection pool in an Axum app

While trying to get this up and running, I was looking into how I could properly set up a connection pool. It required a little bit of digging, so I figured I may as well pitch in and add an example. I wanted to use it with Axum in particular, so I've also included an example for that too. Getting this setup required some shifts around the repo.

From my commit message:

To get this to work, we also need to move the code from src/ to its own package in memcache. This is because we're adding a complex example with axum, which includes its own Cargo.toml. This has other effects: it means that you can't add single file examples anymore. It must be a crate. This is the same decision used in the Axum repo, and is just a current limitation of cargo.

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see also: https://github.com/tokio-rs/axum/tree/main/examples

[aisk]

Adding examples is good, but I don't think it's a good reason to change the whole project structure. The axum project has multiple crates besides the example, so it is reasonable.