Add Async Support

[Electron100]

Add async support. There are still a few outstanding gaps and potential issues which are tracked in async_checklist.md. We'll wait to release 0.8 for those to be resolved.

Addresses #13

[jayvdb]

New Rust 1.82 lint rust-lang/rust#129207
It is allow by default - our CI is causing it to be an error when running rustdoc.

error: elided lifetime has a name
Error:    --> butane_core/src/db/dummy.rs:123:59
    |
123 |     async fn transaction(&mut self) -> Result<Transaction<'_>> {
    |                          -                                ^^ this elided lifetime gets resolved as `'life0`
    |                          |
    |                          lifetime `'life0` declared here
    |
    = note: `-D elided-named-lifetimes` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(elided_named_lifetimes)]`

error: elided lifetime has a name
Error:    --> butane_core/src/db/pg.rs:113:59
    |
113 |     async fn transaction(&mut self) -> Result<Transaction<'_>> {
    |                          -                                ^^ this elided lifetime gets resolved as `'life0`
    |                          |
    |                          lifetime `'life0` declared here

error: elided lifetime has a name
Error:   --> butane_core/src/db/mod.rs:94:59
   |
94 |     async fn transaction(&mut self) -> Result<Transaction<'_>>;
   |                          -                                ^^ this elided lifetime gets resolved as `'life0`
   |                          |
   |                          lifetime `'life0` declared here

error: elided lifetime has a name
Error:    --> butane_core/src/db/mod.rs:116:47
    |
116 |     async fn transaction(&mut self) -> Result<Transaction> {
    |                          -                    ^^^^^^^^^^^ this elided lifetime gets resolved as `'life0`
    |                          |
    |                          lifetime `'life0` declared here

error: elided lifetime has a name
Error:    --> butane_core/src/db/mod.rs:289:47
    |
289 |     async fn transaction(&mut self) -> Result<Transaction> {
    |                          -                    ^^^^^^^^^^^ this elided lifetime gets resolved as `'life0`
    |                          |
    |                          lifetime `'life0` declared here

error: elided lifetime has a name
Error:    --> butane_core/src/fkey.rs:100:74
    |
100 |     async fn load<'a>(&'a self, conn: &impl ConnectionMethods) -> Result<&T>
    |                   -- lifetime `'a` declared here                         ^ this elided lifetime gets resolved as `'a`

error: elided lifetime has a name
Error:    --> butane_core/src/fkey.rs:106:79
    |
106 |     async fn load<'a>(&'a self, conn: &impl ConnectionMethodsAsync) -> Result<&T>
    |                   -- lifetime `'a` declared here                              ^ this elided lifetime gets resolved as `'a`

error: elided lifetime has a name
Error:    --> butane_core/src/fkey.rs:123:68
    |
123 |     fn load<'a>(&'a self, conn: &impl ConnectionMethods) -> Result<&T>
    |             -- lifetime `'a` declared here                         ^ this elided lifetime gets resolved as `'a`

IMO we should ignore this warning in the rustdoc step if it cant be easily fixed.

[jayvdb]

two previous comments; but lgtm