WIP: dropshot support for multiple API versions

[ahl]

I think these are most of the steps:

• #[endpoint] macro parsing
• macro creation of versioned endpoints
• route tree creation of multiple versions including detection of overlapping version ranges
• enumeration of versions
• proper routing to versioned handlers
• extraction of OpenAPI document for a particular version

---

Here's my proposal for how we parse versions with the proc macro:

#[endpoint {
    method = GET,
    path = "/testing/demo1",
    // versions = "1.2.3"..,
    // versions = .."1.2.5",
    versions = "1.2.3".."1.2.5",
}]
async fn demo_handler_args_1(..) {}

[davepacheco]

Elaborated on the plan here that we discussed a few weeks ago in #869.